Faris manifold

Vector fields and differential forms William G. Faris September 25, 2008 ii Contents 1 Forms 1 1.1 The dual space . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Differential 1-forms . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3 Ordinary differential equations in two dimensions . . . . . . . . . 4 1.4 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 The exterior derivative 7 2.1 The exterior product . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Differential r-forms . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3 Properties of the exterior derivative . . . . . . . . . . . . . . . . 9 2.4 The integrability condition . . . . . . . . . . . . . . . . . . . . . . 10 2.5 Gradient, curl, divergence . . . . . . . . . . . . . . . . . . . . . . 11 2.6 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3 Integration and Stokes’s theorem 13 3.1 One-dimensional integrals . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Integration on manifolds . . . . . . . . . . . . . . . . . . . . . . . 14 3.3 The fundamental theorem . . . . . . . . . . . . . . . . . . . . . . 15 3.4 Green’s theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.5 Stokes’s theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.6 Gauss’s theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.7 The generalized Stokes’s theorem . . . . . . . . . . . . . . . . . . 17 3.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.9 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4 The divergence theorem 21 4.1 Contraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2 Duality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.3 The divergence theorem . . . . . . . . . . . . . . . . . . . . . . . 23 4.4 Integration by parts . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.5 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 iii iv CONTENTS 5 The metric 27 5.1 Inner product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.2 Riemannian metric . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.3 Gradient and divergence . . . . . . . . . . . . . . . . . . . . . . . 29 5.4 Gradient dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 30 5.5 The Laplace operator . . . . . . . . . . . . . . . . . . . . . . . . 30 5.6 Curl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 5.7 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 6 Length and area 33 6.1 Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 6.2 Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 6.3 Divergence and Stokes theorems . . . . . . . . . . . . . . . . . . 35 Chapter 1 Forms 1.1 The dual space The objects that are dual to vectors are 1-forms. A 1-form is a linear transfor- mation from the n-dimensional vector space Vto the real numbers. The 1-forms also form a vector space V∗of dimension n, often called the dual space of the original space Vof vectors. If αis a 1-form, then the value of αon a vector vcould be written as α(v), but instead of this we shall mainly use α·v. The condition of being linear says that α·(au+bv) =aα·u+bα·v. (1.1) The vector space of all 1-forms is called V∗. Sometimes it is called the dual space ofV. It is important to note that the use of the dot in this context is not meant to say that this is the inner product (scalar product) of two vectors. In Part III of this book we shall see how to associate a form guto a vector u, and the inner product of uwithwwill then be gu·w. There is a useful way to picture vectors and 1-forms. A vector is pictured as an arrow with its tail at the origin of the vector space V. A 1-form is pictured by its contour lines (in two dimensions) or its contour planes (in three dimensions). These are parallel lines or parallel planes that represent when the values of the 1-form are multiples of some fixed small number δ >0. Sometimes it is helpful to indicate which direction is the direction of increase. The value α·vof a 1-form αon a vector vis the value associated with the contour that passes through the head of the arrow. Each contour line is labelled by a numerical value. In practice one only draws contour lines corresponding to multiples of some fixed small numerical value. Since this numerical value is somewhat arbitrary, it is customary to just draw the contour lines and indicate the direction of increase. The contour line passing through the origin has value zero. A more precise specification of the 1-form would give the numerical value associated with at least one other contour line. 1 2 CHAPTER 1. FORMS A scalar multiple cαof a 1-form αhas contour lines with increased or decreased spacing, and possibly with reversed direction of increase. The sum α+βof two 1-forms α, βis defined by adding their values. The sum of two 1-forms may also be indicated graphically by a parallelogram law. The two forms define an array of parallelograms. The contour lines of the sum of the two forms are lines through two (appropriately chosen) corners of the parallelograms. 1.2 Differential 1-forms Adifferential form is a linear transformation from the vector fields to the reals given by α=nX i=1aidxi. (1.2) We identify a vector field vwith the corresponding directional derivative v=nX i=1vi∂ ∂xi. (1.3) The value of αon the vector field vis α·v=nX i=1aivi. (1.4) Ifzis a scalar function on M, then it has a differential given by dz=nX i=1∂z ∂xidxi. (1.5) This is a special kind of differential form. In general, a differential form that is the differential of a scalar is called an exact differential form. Ifzis a smooth function on M, and vis a vector field, then the directional derivative ofzalong vis dz·v=nX i=1vi∂z ∂xi. (1.6) It is another smooth function on M. Theorem 1 (Necessary condition for exactness) Ifα=Pn i=1aidxiis an exact differential form, then its coefficients satisfy the integrability conditions ∂ai ∂xj=∂aj ∂xi. (1.7) When the integrability condition is satisfied, then the differential form is said to be closed . Thus the theorem says that every exact form is closed. 1.2. DIFFERENTIAL 1-FORMS 3 In two dimensions an exact differential form is of the form dh(x, y) =∂h(x, y) ∂xdx+∂h(x, y) ∂ydy. (1.8) Ifz=h(x, y) this can be written in a shorter notation as dz=∂z ∂xdx+∂z ∂ydy. (1.9) It is easy to picture an exact differential form in this two-dimensional case. Just picture contour curves of the function z=h(x, y). These are curves defined byz=h(x, y) =c, where the values of care spaced by some small δ >0. Notice that adding a constant to zis does not change the differential of z. It also does not change the contour curves of z. For determination of the differential form what is important is not the value of the function, since this has an arbitrary constant. Rather it is the spacing between the contour curves that is essential. In this picture the exact differential form should be thought of a closeup view, so that on this scale the contour curves look very much like contour lines. So the differential form at a point depends only on the contour lines very near this point. In two dimensions a general differential form is of the form α=f(x, y)dx+g(x, y)dy. (1.10) The condition for a closed form is ∂g(x, y) ∂x=∂f(x, y) ∂y. (1.11) If the form is not closed, then it is not exact. The typical differential form is not closed. We could also write this as α=p dx+q dy. (1.12) The condition for a closed form is ∂q ∂x=∂p ∂y. (1.13) It somewhat harder to picture a differential 1-form that is not exact. The idea is to draw contour lines near each point that somehow join to form contour curves. However the problem is that these contour curves now must have end points, in order to keep the density of lines near each point to be consistent with the definition of the differential form. Example. A typical example of a differential form that is not exact is y dx. The contour lines are all vertical. They are increasing to the right in the upper half plane, and they are increasing to the left in the lower half plane. However the density of these contour lines must diminish near the xaxis, so that some 4 CHAPTER 1. FORMS of the lines will have end points at their lower ends (in the upper half plane) or at their upper ends (in the lower half plane). A differential form may be expressed in various coordinate systems. Say, for instance, that α=p dx+q dy. (1.14) We may write dx=∂x ∂udu+∂x ∂vdv, (1.15) dy=∂y ∂udu+∂y ∂vdv. (1.16) Inserting this in the expression for the 1-form α, we obtain α=µ∂x ∂up+∂y ∂uq¶ du+µ∂x ∂vp+∂y ∂v¶ dv. (1.17) 1.3 Ordinary differential equations in two di- mensions A classic application of these ideas is ordinary differential equations in the plane. Such an equation is often written in the form p dx+q dy= 0. (1.18) Here p=f(x, y) and q=g(x, y) are functions of x, y. The equation is deter- mined by the differential form p dx+q dy, but two different forms may determine equivalent equations. For example, if µ=h(x, y) is a non-zero scalar, then the form µp dx +µq dy is a quite different form, but it determines an equivalent differential equation. Ifp dx+q dyis exact, then p dx+q dy=dz, for some scalar zdepending onxandy. The solution of the differential equation is then given implicitly by z=c, where cis constant of integration. Ifp dx+q dyis not exact, then one looks for an integrating factor µsuch that µ(p dx+q dy) =dz (1.19) is exact. Once this is done, again the solution of the differential equation is then given implicitly by z=c, where cis constant of integration. Theorem 2 Suppose that α=p dx+q dyis a differential form in two dimen- sions that is non-zero near some point. Then αhas a non-zero integrating factor µnear the point, so µα=dsfor some scalar. This theorem follows from the theory of solutions of ordinary differential equations. Finding the integrating factor may not be an easy matter. However, there is a strategy that may be helpful. 1.3. ORDINARY DIFFERENTIAL EQUATIONS IN TWO DIMENSIONS 5 Recall that if a differential form is exact, then it is closed. So if µis an integrating factor, then ∂µp ∂y−∂µq ∂x= 0. (1.20) This condition may be written in the form p∂µ ∂y−q∂µ ∂x+µ∂p ∂y−∂q ∂x¶ µ= 0. (1.21) Say that by good fortune there is an integrating factor µthat depends only onx. Then this gives a linear ordinary differential equation for µthat may be solved by integration. Example. Consider the standard problem of solving the linear differential equation dy dx=−ay+b, (1.22) where a, bare functions of x. Consider the differential form ( ay−b)dx+dy. Look for an integrating factor µthat depends only on x. The differential equation for µis−dµ/dx =aµ. This has solution µ=eA, where Ais a function of xwith dA/dx =a. Thus eA(ay−b)dx+eAdy=d(eAy−S), (1.23) where Sis a function of xwith dS/dx =eAb. So the solution of the equation is y=e−A(S+c). Theorem 3 Consider a differential form α=p dx+q dyin two dimensions. Suppose that near some point αis not zero. Suppose also that αis not closed near this point. Then near this point there is a new coordinate system u, vwith α=u dv. The proof is to note that if α=p dx+q dyis not zero, then it has a non-zero integrating factor with µα=dv. So we can write α=u dv, where u= 1/µ. Since u dv=p dx+q dy, we have u∂v/∂x =pandu∂v/∂y =q. It follows that ∂q/∂x −∂p/∂y =∂u/∂x∂v/∂y −∂u/∂y∂v∂x . Since this is non-zero, the inverse function theorem shows that this is a legitimate change of coordinates. The situation is already considerably more complicated in three dimensions, the canonical form is relatively complicated. The differential equations book by Ince [9] treats this situation. 6 CHAPTER 1. FORMS 1.4 Problems 1.Exact differentials. Is ( x2+y2)dx+ 2xy dy an exact differential form? If so, write it as the differential of a scalar. 2.Exact differentials. Is (1 + ex)dy+ex(y−x)dxan exact differential? If so, write it as the differential of a scalar. 3.Exact differentials. Is eydx+x(ey+ 1)dyan exact differential? If so, write it as the differential of a scalar. 4.Constant differential forms. A differential form usually cannot be trans- formed into a constant differential form, but there are special circum- stances when that can occur. Is it possible to find coordinates u and v near a given point (not the origin) such that −y dx+x dy=du? (1.24) 5.Constant differential forms. A differential form usually cannot be trans- formed into a constant differential form, but there are special circum- stances when that can occur. Is it possible to find coordinates u and v near a given point (not the origin) such that −y x2+y2dx+x x2+y2dy=du? (1.25) 6.Ordinary differential equations. Solve the differential equation ( xy2+ y)dx−x dy= 0 by finding an integrating factor that depends only on y. Chapter 2 The exterior derivative 2.1 The exterior product LetV×Vbe the set of ordered pairs u,vof vectors in V. A2-form σis an anti-symmetric bilinear transformation σ:V×V→R. Thus for each fixed v the function u/mapsto→σ(u,v) is linear, and for each fixed uthe function v/mapsto→(u,v) is linear. Furthermore, σ(u,v) =−σ(v,u). The vector space of all 2-forms is denoted Λ2V∗. It is a vector space of dimension n(n−1)/2. A 2-form has a geometric interpretation. First consider the situation in the plane. Given two planar 2-forms, at least one of them is a multiple of the other. So the space of planar 2-forms is one-dimensional. However we should not think of such a 2-form as a number, but rather as a grid of closely spaced points. The idea is that the value of the 2-form is proportional to the number of points inside the parallelogram spanned by the two vectors. The actual way the points are arranged is not important; all that counts is the (relative) density of points. Actually, to specify the 2-form one needs to specify not only the points but also an orientation, which is just a way of saying that the sign of the answer needs to be determined. In three-dimensional space one can think of parallel lines instead of points. The space of 2-forms in three-dimensional space has dimension 3, because these line can have various directions as well as different spacing. The value of the 2- form on a pair of vectors is proportional to the number of lines passing through the parallelogram spanned by the two vectors. Again, there is an orientation associated with the line, which means that one can perhaps think of each line as a thin coil wound in a certain sense. The sum of two 2-forms may be given by a geometrical construction that somewhat resembles vector addition. Theexterior product (or wedge product) α∧βof two 1-forms is a 2-form. This is defined by (α∧β)(u,v) = det·α·uα·v β·uβ·v¸ = (α·u)(β·v)−(β·u)(α·v).(2.1) 7 8 CHAPTER 2. THE EXTERIOR DERIVATIVE Notice that α∧β=−β∧α. In particular α∧α= 0. The exterior product of two 1-forms has a nice geometrical interpretation. On two dimensions each of the two 1-forms is given by a family of parallel lines. The corresponding 2-form consists of the points at the intersection of these lines. In three dimensions each of the two 1-forms is given by a collection of parallel planes. The corresponding 2-form consists of the lines that are the intersections of these planes. In a similar way, one can define a 3-form τas an alternating trilinear function from ordered triples of vectors to the reals. In three dimensions a 3-form is pictured by a density of dots. One way of getting a 3-form is by taking the exterior product of three 1- forms. The formula for this is (α∧β∧γ)(u,v,w) = det2 4α·uα·vα·w β·uβ·vβ·w γ·uγ·vγ·w3 5 (2.2) In a similar way one can define r-forms on an ndimensional vector space V. The space of such r-forms is denoted ΛrV∗, and it has dimension given by the binomial coefficient¡n r¢ . It is also possible to take the exterior product of r1-forms and get an r-form. The formula for this multiple exterior product is again given by a determinant. The algebra of differential forms is simple. The sum of two r-forms is an r form. The product of an r-form and an s-form is an r+s-form. This multipli- cation satisfies the associative law. It also satisfies the law β∧α= (−1)rsα∧β, (2.3) where αis an r-form and βis an s-form. For instance, if r=s= 1, then α∧β=−β∧α. On the other hand, if r= 1, s= 2, then αβ=βα. 2.2 Differential r-forms One can also have differential r-forms on a manifold. For instance, on three dimensions one might have a differential 2-form such as σ=a dy∧dz+b dz∧dx+c dx∧dy. (2.4) Here x, y, z are arbitrary coordinates, and a, b, c are smooth functions of x, y, z . Similarly, in three dimensions a typical 3-form might have the form τ=s dx∧dy∧dz. (2.5) Notice that these forms are created as linear combinations of exterior products of 1-forms. Since these expressions are so common, it is customary in many contexts to omit the explicit symbol for the exterior product. Thus the forms might be written σ=a dy dz +b dz dx +c dx dy (2.6) 2.3. PROPERTIES OF THE EXTERIOR DERIVATIVE 9 and τ=s dx dy dz. (2.7) The geometric interpretation of such forms is quite natural. For instance, in the three dimensional situation of these examples, a 1-form is represented by a family of surfaces, possibly ending in curves. Near each point of the manifold the family of surfaces looks like a family of parallel contour planes. A 2-form is represented by a family of curves, possibly ending in points. Near each point of the manifold they look like a family of parallel lines. Similarly, a 3-form is represented by a cloud of points. While the density of points near a given point of the manifold is constant, at distant points of the manifold the densities may differ. 2.3 Properties of the exterior derivative The exterior derivative of an r-form αis an r+ 1-form dα. It is defined by taking the differentials of the coefficients of the r-form. For instance, for the 1-form α=p dx+q dy+r dz (2.8) the differential is dα=dp dx +dq dy +dr dz. (2.9) This can be simplified as follows. First, note that dp=∂p ∂xdx+∂p ∂ydy+∂p ∂zdz. (2.10) Therefore dp dx =∂p ∂ydy dx +∂p ∂zdz dx =−∂p ∂ydx dy +∂p ∂zdz dx. (2.11) Therefore, the final answer is dα=d(p dx+q dy+r dz) =µ∂r ∂y−∂q ∂z¶ dy dz +µ∂p ∂z−∂r ∂x¶ dz dx +µ∂q ∂x−∂p ∂y¶ dx dy. (2.12) Similarly, suppose that we have a 2-form σ=a dy dz +b dz dx +c dx dy. (2.13) Then dσ=da dy dz +db dz dx +dc dx dy =∂a ∂xdx dy dz +∂b ∂ydy dz dx +∂c ∂zdz dx dy. (2.14) This simplifies to dσ=d(a dy dz +b dz dx +c dx dy ) =µ∂a ∂x+∂b ∂y+∂c ∂z¶ dx dy dz. (2.15) 10 CHAPTER 2. THE EXTERIOR DERIVATIVE The geometrical interpretation of the exterior derivative is natural. Consider first the case of two dimension. If αis a 1-form, then it is given by a family of curves, possibly with end points. The derivative dαcorresponds to these end points. They have an orientation depending on which end of the curve they are at. In three dimensions, if αis a 1-form, then it is given by contour surfaces, possibly ending in curves. The 2-form dαis given by the curves. Also, if σis a 2-form, then it is given by curves that may terminate. Then dσis a 3-form represented by the termination points. The exterior derivative satisfies various general properties. The exterior derivative of an r-form is an r+ 1 form. There is a product rule d(α∧β) =dα∧β+ (−1)rα∧dβ, (2.16) where αis an r-form and βis an s-form. The reason for the ( −1)ris that the dhas to be moved past the rform, and this picks up rfactors of −1. Another important property is that applying the exterior derivative twice always gives zero, that is, for an arbitrary s-form βwe have ddβ= 0. (2.17) 2.4 The integrability condition This last property has a geometrical interpretation. Take for example a scalar s. Its differential is α=ds, which is an exact differential. Therefore dsis rep- resented by curves without end points (two dimensions) or by surfaces without ending curves (three dimensions). This explains why dα=dds= 0. Similarly, consider a 1-form αin three dimensions. Its differential is a 2-form σ=dα. The 1-form αis represented by surfaces, which may terminate in closed curves. These closed curves represent the 2 form dα. Since they have no end points, we see that dσ=ddα= 0. In general, if dβ= 0, then we say that βis a closed form. If β=dα, we say that βis an exact form. The general fact is that if βis exact, then βis closed. The condition that dβ= 0 is called the integrability condition, since it is necessary for the possibility that βcan be integrated to get αwith β=dα. Example. Consider the 2-form y dx. This is represented by vertical lines that terminate at points in the plane. The density of these lines is greater as one gets farther from the xaxis. The increase is to the right above the xaxis, and it is to the left below the yaxis. The differential of y dxisdy dx =−dx dy . This 2-form represents the cloud of terminating points, which has a uniform density. The usual convention that the positive orientation is counterclockwise. So the orientations of these source points are clockwise. This is consistent with the direction of increase along the contours lines. 2.5. GRADIENT, CURL, DIVERGENCE 11 2.5 Gradient, curl, divergence Consider the case of three dimensions. Anyone familiar with vector analysis will notice that if sis a scalar, then the formula for dsresembles the formula for the gradient in cartesian coordinates. Similarly, if αis a 1-form, then the formula fordαresembles the formula for the curl in cartesian coordinates. The formula dds= 0 then corresponds to the formula curl grad s= 0. In a similar way, if σis a 2-form, then the formula for dσresembles the formula for the divergence in cartesian coordinates. The formula ddα= 0 then corresponds to the formula div curl v= 0. There are, however, important distinctions. First, the differential form for- mulas take the same form in arbitrary coordinate systems. This is not true for the formulas for the divergence, curl, and divergence. The reason is that the usual definitions of divergence, curl, and divergence are as operations on vector fields, not on differential forms. This leads to a much more complicated theory, except for the very special case of cartesian coordinates on Euclidean space. We shall examine this issue in detail in the third part of this book. Second, the differential form formulas have natural formulations for mani- folds of arbitrary dimension. While the gradient and divergence may also be formulated in arbitrary dimensions, the curl only works in three dimensions. This does not mean that notions such as gradient of a scalar (a vector field) or divergence of a vector field (a scalar) are not useful and important. Indeed, in some situations they play an essential role. However one should recognize that these are relatively complicated objects. Their nature will be explored in the second part of this book (for the divergence) and in the third part of this book (for the gradient and curl). The same considerations apply to the purely algebraic operations, at least in three dimensions. The exterior product of two 1-forms resembles in some way the cross product of vectors, while the exterior product of a 1-form and a 2-form resembles a scalar product of vectors. Thus the wedge product of three 1-forms resembles the triple scalar product of vector analysis. Again these are not quite the same thing, and the relation will be explored in the third part of this book. 12 CHAPTER 2. THE EXTERIOR DERIVATIVE 2.6 Problems 1.Say that the differential 1-form α=p dx+q dy+r dzhas an integrating factor µ/negationslash= 0 such that µα=ds. Prove that α∧dα= 0. Also, express this condition as a condition on p, q, r and their partial derivatives. 2.Show that α=dz−y dx−dyhas no integrating factor. 3.Show that the differential 1-form α=yz dx +xz dy +dzpasses the test for an integrating factor. 4.In the previous problem it might be difficult to guess the integrating factor. Show that µ=exyis an integrating factor, and find swith µα=ds. 5.The differential 2-form ω= (2xy−x2)dx dy is of the form ω=dα, where αis a 1-form. Find such an α. Hint: This is too easy; there are many solutions. 6.The differential 3-form σ= (yz+x2z2+ 3xy2z)dx dy dz is of the form σ=dω, where ωis a 2-form. Find such an ω. Hint: Many solutions. 7.Letσ=xy2z dy dz −y3z dz dx +(x2y+y2z2)dx dy . Show that this 2-form σsatisfies dσ= 0. 8.The previous problem gives hope that σ=dαfor some 1-form α. Find such an α. Hint: This may require some experimentation. Try αof the form α=p dx+q dy, where p, qare functions of x, y, z . With luck, this may work. Remember that when integrating with respect to zthe constant of integration is allowed to depend on x, y. Chapter 3 Integration and Stokes’s theorem 3.1 One-dimensional integrals A one-dimensional manifold Cis described by a single coordinate t. Consider an interval on the manifold bounded by t=aandt=b. There are two possible orientations of this manifold, from t=atot=b, or from t=btot=a. Suppose for the sake of definiteness that the manifold has the first orientation. Then the differential form f(t)dthas the integral Z Cf(t)dt=Zt=b t=af(t)dt. (3.1) Ifsis another coordinate, then tis related to sbyt=g(s). Furthermore, there are numbers p, qsuch that a=g(p) and b=g(q). The differential form is thus f(t)dt=f(g(s))g/prime(s)ds. The end points of the manifold are s=pand s=q. ThusZ Cf(t)dt=Zs=q s=pf(g(s))g/prime(s)ds. (3.2) The value of the integral thus does not depend on which coordinate is used. Notice that this calculation depends on the fact that dt/ds =g/prime(s) is non- zero. However we could also consider a smooth function uon the manifold that is not a coordinate. Several points on the manifold could give the same value of u, and du/ds could be zero at various places. However we can express u=h(s) anddu/ds =h/prime(s) and define an integral Z Cf(u)du=Zs=q s=pf(h(s))h/prime(s)ds. (3.3) Thus the differential form f(u)dualso has a well-defined integral on the mani- fold, even though uis not a coordinate. 13 14 CHAPTER 3. INTEGRATION AND STOKES’S THEOREM 3.2 Integration on manifolds Next look at the two dimensional case. Say that we have a coordinate system x, yin a two-dimensional oriented manifold. Consider a region Rbounded by curves x=a,x=b, and by y=c,y=d. Suppose that the orientation is such that one goes around the region in the order a, bthen c, dthen b, athen d, c. Then the differential form f(x, y)dx dy has integral Z Rf(x, y)dx dy =Zd c"Zb af(x, y)dx# dy=Zb a"Zd cf(x, y)dy# , dx. (3.4) The limits are taken by going around the region in the order given by the orientation, first a, bthen c, d. We could also have taken first b, athen d, cand obtained the same result. Notice, by the way, that we could also define an integral with dy dx in place ofdx dy . This would be Z Rf(x, y)dy dx =Za b"Zd cf(x, y)dy# dx=Zd c·Za bf(x, y)dx¸ , dy. (3.5) The limits are taken by going around the region in the order given by the orientation, first c, dthen b, a. We could also have taken d, cthen a, band obtained the same result. This result is precisely the negative of the previous result. This is consistent with the fact that dy dx =−dx dy . These formula have generalizations. Say that the region is given by letting xgo from atobandyfrom h(x) tok(x). Alternatively, it might be given by letting ygo from ctodandxfrom p(y) toq(y). This is a more general region than a rectangle, but the same kind of formula applies: Z Rf(x, y)dx dy =Zd c"Zq(y) p(y)f(x, y)dx# dy=Zb a"Zk(x) h(x)f(x, y)dy# , dx. (3.6) There is yet one more generalization, to the case where the differential form isf(u, v)du dv , but u, vdo not form a coordinate system. Thus, for instance, the 1-form dumight be a multiple of dvat a certain point, so that du dv would be zero at that point. However we can define the integral by using the customary change of variable formula: Z Rf(u, v)du dv =Z Rf(u, v)µ∂u ∂x∂v ∂y−∂v ∂x∂u ∂y¶ dx dy. (3.7) In fact, since du=∂u/∂x dx +∂u/∂y dy anddv=∂v/∂x dx +∂v/∂y dy , this is just saying that the same differential form has the same integral. In fact, we could interpret this integral directly as a limit of sums involving only the u, vincrements. Partition the manifold by curves of constant xand 3.3. THE FUNDAMENTAL THEOREM 15 constant y. This divides the manifold into small regions that look something like parallelograms. Then we could write this sum as Z Rf(u, v)du dv≈X f(u, v) (∆ux∆vy−∆vx∆uy). (3.8) Here the sum is over the parallelograms. The quantity ∆ uxis the increment in ufrom xtox+ ∆x, keeping yfixed, along one side of the parallelogram. The quantity ∆ vyis the increment in vfrom ytoy+ ∆y, keeping yfixed, along one side of the parallelogram. The other quantities are defined similarly. The u, v value is evaluated somewhere inside the parallelogram. The minus sign seems a bit surprising, until one realizes that going around the oriented boundary of the parallelogram the proper orientation makes a change from xtox+ ∆xfollowed by a change from ytoy+∆y, or a change from ytoy+∆yfollowed by a change from x+ ∆xtox. So both terms have the form ∆ u∆v, where the changes are now taken along two sides in the proper orientation, first the change in u, then the change in v. 3.3 The fundamental theorem The fundamental theorem of calculus says that for every scalar function swe have Z Cds=s(Q)−s(P). (3.9) Here Cis an oriented path from point Pto point Q. Notice that the result does not depend on the choice of path. This is because dsis an exact form. As an example, we can take a path in space. Then ds=∂s/∂x dx + ∂s/∂y dy +∂s/∂z dz . So Z Cds=Z C∂s ∂xdx+∂s ∂ydy+∂z ∂zdz=Z Cµ∂s ∂xdx dt+∂s ∂ydy dt+∂z ∂zdz dt¶ dt.(3.10) By the chain rule this is just Z Cds=Z Cds dtdt=s(Q)−s(P). (3.11) 3.4 Green’s theorem The next integral theorem is Green’s theorem. It says that Z Rµ∂q ∂x−∂p ∂y¶ dx dy =Z ∂Rp dx+q dy. (3.12) Here Ris an oriented region in two dimensional space, and ∂Ris the curve that is its oriented boundary. Notice that this theorem may be stated in the succinct form Z Rdα=Z ∂Rα. (3.13) 16 CHAPTER 3. INTEGRATION AND STOKES’S THEOREM The proof of Green’s theorem just amounts to applying the fundamental theorem of calculus to each term. Thus for the second term one applies the fundamental theorem of calculus in the xvariable for fixed y. Z R∂q ∂xdx dy =Zd c"Z Cyq dx# dy=Zd c£ q(C+ y)−q(C− y)¤ dy. (3.14) This isZd cq(C+ y)dy+Zc dq(C− y)dy=Z ∂Rq dy. (3.15) The other term is handled similarly, except that the fundamental theorem of calculus is applied with respect to the xvariable for fixed y. Then such regions can be pieced together to give the general Green’s theorem. 3.5 Stokes’s theorem The most common version of Stokes’s theorem says that for a oriented two dimensional surface Sin a three dimensional manifold with oriented boundary curve ∂Swe have Z Sµ∂r ∂y−∂q ∂z¶ dy dz +µ∂p ∂z−∂r ∂x¶ dz dx +µ∂q ∂x−∂p ∂y¶ dx dy =Z ∂S(p dx+q dy+r dz). (3.16) Again this has the simple form Z Sdα=Z ∂Sα. (3.17) This theorem reduces to Green’s theorem. The idea is to take coordinates u, von the surface Sand apply Green’s theorem in the u, vcoordinates. In the theorem the left hand side is obtained by taking the form p dx+q dy+r dzand applying dto it. The key observation is that when the result of this is expressed in the u, vcoordinates, it is the same as if the form p dx+q dy+r dzwere first expressed in the u, vcoordinates and then dwere applied to it. In this latter form Green’s theorem applies directly. Here is the calculation. To make it simple, consider only the p dx term. Then taking dgives d(p dx) =µ∂p ∂xdx+∂p ∂ydy+∂p ∂zdz¶ dx=∂p ∂zdz dx−∂p ∂ydx dy. (3.18) Inu, vcoordinates this is d(p dx) =·∂p ∂zµ∂z ∂u∂x ∂v−∂x ∂u∂z ∂v¶ −∂p ∂yµ∂x ∂u∂y ∂v−∂y ∂u∂x ∂v¶¸ du dv. (3.19) There are four terms in all. 3.6. GAUSS’S THEOREM 17 Now we do it in the other order. In u, vcoordinates we have p dx=p∂x ∂udu+p∂x ∂vdv. (3.20) Taking dof this gives dµ p∂x ∂udu+p∂x ∂vdv¶ =·∂ ∂uµ p∂x ∂v¶ −∂ ∂vµ p∂x ∂u¶¸ du dv. (3.21) The miracle is that the second partial derivatives cancel. So in this version dµ p∂x ∂udu+p∂x ∂vdv¶ =·∂p ∂u∂x ∂v−∂p ∂v∂x ∂u¸ du dv. (3.22) Now we can express ∂p/∂u and∂p/∂v by the chain rule. This gives at total of six terms. But two of them cancel, so we get the same result as before. 3.6 Gauss’s theorem LetWbe an oriented three dimensional region, and let ∂Wbe the oriented surface that forms its boundary. Then Gauss’s theorem states that Z Wµ∂a ∂x+∂b ∂y+∂c ∂z¶ dx dy dz =Z ∂Wa dy dz +b dz dx +c dx dy. (3.23) Again this has the form Z Wdσ=Z ∂Wσ, (3.24) where now σis a 2-form. The proof of Gauss’s theorem is similar to the proof of Green’s theorem. 3.7 The generalized Stokes’s theorem The generalized Stoke’s theorem says that Z Ωdω=Z ∂Ωω. (3.25) Here ωis a (k−1)-form, and dωis ak-form. Furthermore, Ω is a kdimensional region, and ∂Ω is its ( k−1)-dimensional oriented boundary. The forms may be expressed in arbitrary coordinate systems. 3.8 References A classic short but rigorous account of differential forms is given in the book of Spivak [15]. The book by Agricola and Friedrich [1] gives a more advanced 18 CHAPTER 3. INTEGRATION AND STOKES’S THEOREM treatment. Other books on differential forms include those by Cartan [2], do Carmo [3], Edelen [4], Flanders [7], Screiber [14], and Weintraub [17]. There are also advanced calculus texts by Edwards [5] and by Hubbard and Hub- bard [8]. There are many sources for tensor analysis; a classical treatment may be found in Lovelock and Rund [10]. There is a particularly unusual and sophis- ticated treatment in the book of Nelson [12]. Differential forms are seen to be special kinds of tensors: covariant alternating tensors. The most amazing reference that this author has encountered is an elemen- tary book by Weinreich [16]. He presents the geometric theory of differential forms in pictures, and these pictures capture the geometrical essence of the sit- uation. The principal results of the theory are true by inspection. However his terminology is most unusual. He treats only the case of dimension three. Thus he has the usual notion of covariant 1-form, 2-form, and 3-form. In his termi- nology the corresponding names for these are stack, sheaf, and scalar density (or swarm). There are also corresponding contravariant objects corresponding to what are typically called 1-vector, 2-vector (surface element), and 3-vector (volume element). The names in this case are arrow, thumbtack, and scalar capacity. The correspondence between his objects and the usual tensors may actually be slightly more complicated than this, but the intent is certainly to explicate the usual calculus geometrically. In particular, he gives geometric ex- planations of the usual algebraic and differential operations in all these various cases. 3.9. PROBLEMS 19 3.9 Problems 1.LetCbe the curve x2+y2= 1 in the first quadrant from (1 ,0) to (0 ,1). Evaluate Z Cxy dx + (x2+y2)dy. (3.26) 2.LetCbe a curve from (2 ,0) to (0 ,3). Evaluate Z C2xy dx + (x2+y2)dy. (3.27) 3.Consider the problem of integrating the differential form α=−y x2+y2dx+x x2+y2dy (3.28) from (1 ,0) to ( −1,0) along some curve avoiding the origin. There is are an infinite set of possible answers, depending on the curve. Describe all such answers. 4.LetRbe the region x2+y2≤1 with x≥0 and y≥0. Let ∂Rbe its boundary (oriented counterclockwise). Evaluate directly Z ∂Rxy dx + (x2+y2)dy. (3.29) 5.This continues the previous problem. Verify Green’s theorem in this spe- cial case, by explicitly calculating the appropriate integral over the region R. 6.Let α=−y dx+x dy+xy dz. (3.30) Fixa >0. Consider the surface Sthat is the hemisphere x2+y2+z2= a2with z≥0. Integrate αover the boundary ∂Sof this surface (a counterclockwise circle in the x, yplane). 7.This continues the previous problem. Verify Stokes’s theorem in this spe- cial case, by explicitly calculating the appropriate integral over the surface S. 8.Letσ=xy2z dy dz −y3z dz dx + (x2y+y2z2)dx dy . Integrate σover the sphere x2+y2+z2=a2. Hint: This should be effortless. 20 CHAPTER 3. INTEGRATION AND STOKES’S THEOREM Chapter 4 The divergence theorem 4.1 Contraction There is another operation called interior product (or contraction). In the case of interest to us, it is a way of defining the product of a vector with a k-form to get a k−1 form. We shall mainly be interested in the case when k= 1,2,3. When k= 1 this is already familiar. For a 1-form αthe interior product u⌋αis defined to be the scalar α·v. The interior product of a vector uwith a 2-form σis a 1-form u⌋σ. It is defined by (u⌋σ)·v=σ(u,v). (4.1) This has a nice picture in two dimensions. The vector uis an arrow. In two dimensions the 2-form σis given by a density of points. The contour lines of the interior product 1-form are parallel to the arrow. The get them, arrange the points defining the 2-form to be spaced according to the separation determined by the arrow (which may require some modification in the other direction to preserve the density). Then take the contour lines to be spaced according to the new arrangement of the points. These contour lines are the contour lines corresponding to the interior product 1-form. In three dimensions the 2-form σis given by lines. The arrow uand the lines determining σdetermine a family of parallel planes. To get these contour planes, do the following. Arrange the lines that determine σto be spaced according to the separation determined by the arrow (which may require some modification in the other direction to preserve the density). Then take the contour planes to be spaced according to the new separation between the lines. The resulting planes are the contour planes of the interior product 1-form. The interior product u⌋ωof a vector uwith a 3-form ωis a 2-form u⌋ω. It is defined by (u⌋ω)(v,w) =ω(u,v,w). (4.2) (The case of a general r-form is similar.) 21 22 CHAPTER 4. THE DIVERGENCE THEOREM The picture is similar. Consider three dimensions. The vector uis an arrow, and the associated 2-form u⌋ωis given by lines that are parallel to the arrow. To get these contour lines, do the following. Arrange the points that determine ωto be spaced according to the separation determined by the arrow. Then take the contour lines to be spaced according to the new separation between the points. One interesting property of the interior product is that if αis an r-form and βis an s-form, then u⌋(α∧β) = (u⌋α)∧β+ (−1)rα∧(u⌋β). (4.3) This is a kind of triple product identity. In particular, we may apply this when r= 1 and s=n. Since βis an n-form, it follows that α∧β= 0. Hence we have in this special case (α·u)β=α∧(u⌋β). (4.4) Another application is with two 1-forms βandγ. In this case it gives a⌋(β∧γ) = (β·a)γ−(γ·a)β. (4.5) So the interior product of a vector with β∧γis a linear combination of βand γ. Later we shall see the connection with classical vector algebra in three di- mensions. The exterior product β∧γis an analog of the cross product, while α∧β∧γis an analog of the triple scalar product. The combination −a⌋(β∧γ) will turn out to be an analog of the triple vector product. 4.2 Duality Consider an n-dimensional manifold. The new feature is a given n-form, taken to be never zero. We denote this form by vol. In coordinates it is of the form vol =√g du 1···dun. (4.6) This coefficient√gdepends on the coordinate system. The choice of the no- tation√gfor the coefficient will be explained in the following chapter. (Then√gwill be the square root of the determinant of the matrix associated with the Riemannian metric for this coordinate system.) The most common examples of volume forms are the volume in vol = dx dy dz in cartesian coordinates and the same volume vol = r2sin(θ)dr dθ dφ in spherical polar coordinates. The convention we are using for spherical polar coordinates is that θis the co-latitude measured from the north pole, while φis the longitude. We see from these coordinates that the√gfactor for cartesian coordinates is 1, while the√gfactor for spherical polar coordinates is r2sin(θ). In two dimensions it is perhaps more natural to call this area. So in cartesian coordinates area = dx dy , while in polar coordinates area = r dr dφ . 4.3. THE DIVERGENCE THEOREM 23 For each scalar field sthere is an associated n-form svol. The scalar field and the n-form determine each other in an obvious way. They are said to be dual to each other, in a certain special sense. For each vector field vthere is an associated n−1 form given by v⌋vol. The vector field and the n−1 form are again considered to be dual to each other, in this same sense. If vis a vector field, then v⌋vol might be called the corresponding flux. It is an n−1 form that describes how much vis penetrating a given n−1 dimensional surface. In two dimensions a vector field is of the form u=a∂ ∂u+b∂ ∂v. (4.7) The area form is area =√g du dv. (4.8) The corresponding flux is u⌋area =√g(a dv−b du). (4.9) In three dimensions a vector field is of the form u=a∂ ∂u+b∂ ∂v+c∂ ∂w. (4.10) The volume form is vol =√g du dv dw. (4.11) The corresponding flux is √g(a dv dw +b dw du +c du dv ). (4.12) 4.3 The divergence theorem The divergence of a vector field vis defined to be the scalar div vsuch that d(u⌋vol) = div uvol. (4.13) In other words, it is the dual of the differential of the dual. The general divergence theorem then takes the form Z Wdivuvol =Z ∂Wu⌋vol. (4.14) In two dimensions the divergence theorem says that Z R1√gµ∂√ga ∂u+∂√gb ∂v¶ area =Z ∂R√g(a dv−b du). (4.15) Here the area form is√g du dv , where the particular form of√gis that associ- ated with the u, vcoordinate system. Notice that the coefficients in the vector 24 CHAPTER 4. THE DIVERGENCE THEOREM field are expressed with respect to a coordinate basis. We shall see in the next part of this book that this is not the only possible choice. A marvellous application of the divergence theorem in two dimensions is the formula Z Rdx dy =1 2Z ∂Rx dy−y dx. (4.16) This says that one can determine the area by walking around the boundary. It is perhaps less mysterious when one realizes that x dy−y dx=r2dφ. In three dimensions the divergence theorem says that Z W1√gµ∂√ga ∂u+∂√gb ∂v+∂√gc ∂w¶ vol =Z ∂W√g(a dv dw +b dw du +c du dv ). (4.17) Here the volume form is√g du dv dw , where the particular form of√gis that associated with the u, v, w coordinate system. Again the coefficients a, b, c of the vector field are expressed in terms of the coordinate basis vectors ∂/∂u, ∂/∂v, ∂/∂w . This is the the only possible kind of basis for a vector field, so in some treat- ments the formulas will appear differently. They will be ultimately equivalent in terms of their geometrical meaning. The divergence theorem says that the integral of the divergence of a vector field over Wwith respect to the volume is the integral of the flux of the vector field across the bounding surface ∂W. A famous application in physics is when the vector field represents the electric field, and the divergence represents the density of charge. So the amount of charge in the region determines the flux of the electric field through the boundary. 4.4 Integration by parts An important identity for differential forms is d(sω) =ds∧ω+sdω. (4.18) This gives an integration by parts formula Z Wds∧ω+Z Wsdω=Z ∂Wsω. (4.19) Apply this to ω=u⌋vol and use ds∧u⌋vol = ds·uvol. This gives the divergence identity div (su) =ds·u+sdivu. (4.20) From this we get another important integration by parts identity Z Wds·uvol +Z Wsdivuvol =Z ∂Wsu⌋vol. (4.21) 4.5. PROBLEMS 25 4.5 Problems 1.Letr2=x2+y2+z2, and let v=1 r3µ x∂ ∂x+y∂ ∂y+z∂ ∂z¶ . (4.22) Let vol = dx dy dz . Show that σ=v⌋vol =1 r3(x dy dz +y dz dx +z dx dy ). (4.23) 2.In the preceding problem, show directly that dσ= 0 away from r= 0. 3.Find σin spherical polar coordinates. Hint: This can be done by blind computation, but there is a better way. Express vin spherical polar coordinates, using Euler’s theorem r∂ ∂r=x∂ ∂x+y∂ ∂y+z∂ ∂z. (4.24) Then use vol = r2sin(θ)dr dθ dφ to calculate σ=v⌋vol. 4.In the preceding problem, show that dσ= 0 away from r= 0 by a spherical polar coordinate calculation. 5.LetSbe the sphere of radius a >0 centered at the origin. Calculate the integral of σoverS. 6.LetQbe the six-sided cube with side lengths 2 Lcentered at the origin. Calculate the integral of σoverQ. Hint: Given the result of the preceding problem, this should be effortless. 26 CHAPTER 4. THE DIVERGENCE THEOREM Chapter 5 The metric 5.1 Inner product Aninner product on a vector space Vis a real function gthat takes a pair of input vectors u,vand produces a number gu·v. It is required to be a bilinear, symmetric, positive, non-degenerate form. That is, it satisfies the following axioms: 1.The form is bilinear: The function gu·vis linear in uand also linear in v. 2.The form is symmetric: gu·v=gv·u. 3.The form is non-degenerate: gu·u= 0 implies u= 0. 4.The form is positive: gu·u≥0, An inner product gdefines a linear transformation g:V→V∗. That is, the value of gonuinVis the linear function from Vto the real numbers that sends vtogu·v. Thus guis such a function, that is, an element of the dual space V∗. Since the form gis non-degenerate, the linear transformation gfrom VtoV∗ is an isomorphism of vector spaces. Therefore it has an inverse g−1:V∗→V. Thus if ωis a linear form in V∗, the corresponding vector u=g−1ωis the unique vector usuch that gu·v=ω·v. In short, once one has a given inner product, one has a tool that tends to erase the distinction between a vector space and its dual space. It is worth noting that in relativity theory there is a generalization of the notion of inner product in which the form is not required to be positive. However it still gives such an isomorphism between vector space and dual space. 27 28 CHAPTER 5. THE METRIC 5.2 Riemannian metric A smooth assignment of an inner product for the tangent vectors at each point of a manifold is called a Riemannian metric . It is very convenient to choose coordinates so that the Riemannian metric is diagonal with respect to this coordinate system. In this case it has the form g=h2 1du2 1+h2 2du2 2+···+h2 ndu2 n. (5.1) Here each coefficient hiis a function of the coordinates u1, . . . , u n. The differ- entials is not interpreted in the sense of differential forms. Rather, what this means is that gtakes vector fields to differential forms by gµ a1∂ ∂u1+···an∂ ∂un¶ =a1h2 1du1+···anh2 ndun (5.2) It is not always possible to find such a coordinate system for which the Rieman- nian metric is diagonal. However this can always be done when the dimension n≤3, and it is very convenient to do so. Such a coordinate system is called a system of orthogonal coordinates . See the book by Eisenhart [6] for a discussion of this point. When we have orthogonal coordinates, it is tempting to make the basis vectors have length one. Thus instead of using the usual coordinate basis vectors ∂ ∂uione uses the normalized basis vectors1 hi∂ ∂ui. Similarly, instead of using the usual coordinate differential forms duione uses the normalized differentials hidui. Then gµ a11 h1∂ ∂u1+···an1 hn∂ ∂un¶ =a1h1du1+···anhndun (5.3) When you use the normalized basis vectors, the coefficients do not change. In orthogonal coordinates the volume is given by vol =√g du 1···dun=h1···hndu1∧ ··· ∧ dun. (5.4) A simple example of orthogonal coordinates is that of polar coordinates r, φ in the plane. These are related to cartesian coordinates x, yby x=rcos(φ) (5.5) y=rsin(φ) (5.6) The Riemannian metric is expressed as g=dr2+r2dφ2. (5.7) The normalized basis vectors are∂ ∂rand1 r∂ ∂φ. The normalized basis forms are drandr dφ. The area form is r dr∧dφ. Warning: Even though coordinate forms like dφare closed forms, a normal- ized form like r dφ need not be a closed form. In fact, in this particular case d(rφ) =dr∧dφ/negationslash= 0. 5.3. GRADIENT AND DIVERGENCE 29 Another example of orthogonal coordinates is that of spherical polar coor- dinates r, θ, φ . These are related to cartesian coordinates x, y, z by x=rcos(φ) sin(θ) (5.8) y=rsin(φ) sin(θ) (5.9) z=rcos(θ) (5.10) The Riemannian metric is expressed as g=dr2+r2dθ2+r2sin2(θ)dφ2. (5.11) The normalized basis vectors are∂ ∂rand1 r∂ ∂θand1 rsin(θ)∂ ∂φ. The normalized basis forms are drandr dθandrsin(θ)dφ. The volume form is r2sin(θ)dr∧ dθ∧dφ. In these examples one could always use cartesian coordinates. However there are manifolds that cannot be naturally described by cartesian coordinates, but for which orthogonal coordinates are available. A simple example is the sphere of constant radius a. The Riemannian metric is expressed by g=a2dθ2+a2sin2(θ)dφ2. (5.12) The normalized basis vectors are1 a∂ ∂θand1 asin(θ)∂ ∂φ. The normalized basis forms are a dθandasin(θ)dφ. The area form is a2sin(θ)dθ∧dφ. 5.3 Gradient and divergence Iffis a scalar field, then its gradient is ∇f= grad f=g−1d f. (5.13) Since duis a 1-form, and the inverse of the metric g−1maps 1-forms to vector fields, the gradient ∇fis a vector field. In orthogonal coordinates ∇fhas the form ∇f=nX i=11 h2 i∂f ∂ui∂ ∂ui. (5.14) In terms of normalized basis vectors this has the equivalent form ∇f=nX i=11 hi∂f ∂ui1 hi∂ ∂ui. (5.15) Ifuis a vector field, then its divergence ∇ ·uis a scalar field given by requiring that (divu)vol = ( ∇ ·u)vol = d(u⌋vol). (5.16) 30 CHAPTER 5. THE METRIC Here vol = h1···hndu1∧···dunis the volume form. Say that uhas an expres- sion in terms of normalized basis vectors of the form u=nX i=1ai1 hi∂ ∂ui. (5.17) Then divu=∇ ·u=nX i=11 h1···hn∂ ∂uiµh1···hn hiai¶ . (5.18) 5.4 Gradient dynamics A scalar function fhas both a differential d fand a gradient g−1d f. What can the gradient do that the differential cannot do? Well, the gradient is a vector field, so it has an associated system of differential equations dui dt=1 h2 i∂f ∂ui. (5.19) Along a solution of this equation the function fsatisfies d f dt=nX i=1∂f ∂ui∂ui ∂t=nX i=11 h2 iµ∂f ∂ui¶2 ≥0. (5.20) In more geometrical language this says that d f dt=d f·g−1d f≥0. (5.21) Along every solution fis increasing in time. If instead you want decrease, you can follow the negative of the gradient. 5.5 The Laplace operator The Laplace operator ∇2is defined as ∇2f=∇ · ∇ f. (5.22) This can also be written ∇2f= div grad f. (5.23) In coordinates the Laplacian has the form ∇2f=1 h1···hnnX i=1∂ ∂uiµh1···hn h2 i∂f ∂ui¶ (5.24) For example, in three dimensions with cartesian coordinates it is ∇2f=∂2f ∂x2+∂2f ∂y2+∂2f ∂z2. (5.25) 5.6. CURL 31 In spherical polar coordinates it is ∇2f=1 r2sin(θ)·∂ ∂rr2sin(θ)∂f ∂r+∂ ∂θsin(θ)∂f ∂θ+∂ ∂φ1 sin(θ)∂f ∂φ¸ .(5.26) This is often written ∇2f=1 r2∂ ∂rr2∂f ∂r+1 r2·1 sin(θ)∂ ∂θsin(θ)∂f ∂θ+1 sin2(θ)∂2f ∂φ2¸ . (5.27) 5.6 Curl The remaining objects are in three dimensions. The cross product of two vectors vandwis defined as the unique vector v×wsuch that (v×w)⌋vol = gv∧gw. (5.28) In other words, it is the operation on vectors that corresponds to the exterior product on forms. The curl of a vector field vis defined by (curlv)⌋vol = d(gv). (5.29) The curl has a rather complicated coordinate representation. Say that in a system of orthogonal coordinates v=a1 hu∂ ∂u+b1 hv∂ ∂v+c1 hw∂ ∂w. (5.30) Thus the vector field is expressed in terms of normalized basis vectors. Then gv=ahudu+bhvdv+chwdw. (5.31) So d(gv) =µ∂hwc ∂v−∂hvb ∂w¶ dv∧dw+µ∂hua ∂w−∂hwc ∂u¶ dw∧du+µ∂hvb ∂u−∂hua ∂v¶ du∧dv. (5.32) It follows that curlv=1 hvhwµ∂hwc ∂v−∂hvb ∂w¶1 hu∂ ∂u+1 huhwµ∂hua ∂w−∂hwc ∂u¶1 hv∂ ∂hv+1 huhvµ∂hvb ∂u−∂hua ∂v¶1 hw∂ ∂hw. (5.33) The reason for writing it this way is to express it again in terms of normal- ized basis vectors. Notice also that if we express the derivatives as normalized derivatives, then the expression is reasonably natural. For instance, the first term is 1 /hwtimes the derivative (1 /hv)∂/∂v ) ofhwtimes the coefficient. The only odd thing is that the hwis inside the derivative, while the 1 /hwis outside the derivative. 32 CHAPTER 5. THE METRIC It is easy to see that curl grad f= 0 and that div curl v= 0. Stokes’s theorem says that Z Scurlv⌋vol =Z ∂Sgv. (5.34) Of course, this is just saying that Z Sd(gv) =Z ∂Sgv, (5.35) which is much simpler, since most of the effect of the metric has now cancelled out. 5.7 Problems 1.This problem is three dimensional. Compute the Laplacian of 1 /rvia a cartesian coordinate calculation. 2.This problem is three dimensional. Compute the Laplacian of 1 /rvia spherical polar coordinates. Chapter 6 Length and area 6.1 Length Sometimes it is useful to consider coordinates on a manifold that are not or- thogonal coordinates. The simplest case is that of a two-dimensional manifold. Write the metric as g=E du2+ 2F du dv +G dv2. (6.1) Here E, F, G are functions of u, v. They of course depend on the choice of coordinates. What is required is that E > 0, G > 0 and the determinant EF−G2>0. When F= 0 we are in the case of orthogonal coordinates. One way that such a metric arises is from a surface in three-dimensional space. Suppose the metric is given in orthogonal coordinates x, y, z byh2 xdx2+ h2 ydy2+h2 zdz2. (If one chooses cartesian coordinates, then hx=hy=hz= 1.) The length of a curve is s=Z Cq h2xdx2+h2ydy2+h2zdz2. (6.2) The meaning of this equation is that s=Zb as h2xµdx dt¶2 +h2yµdy dt¶2 +h2zµdz dt¶2 dt, (6.3) where tis a coordinate on the curve, and the end points are where t=aand t=b. Suppose that the curve is in the surface. Then the length is s=Zb as Eµdu dt¶2 + 2Fdu dtdv dt+Gµdv dt¶2 dt. (6.4) Here the coefficients are E=h2 xµ∂x ∂u¶2 +h2 yµ∂y ∂u¶2 +h2 zµ∂z ∂u¶2 , (6.5) 33 34 CHAPTER 6. LENGTH AND AREA and F=h2 x∂x ∂u∂x ∂v+h2 y∂y ∂u∂y ∂v+h2 z∂z ∂u∂z ∂v, (6.6) and G=h2 xµ∂x ∂v¶2 +h2 yµ∂y ∂v¶2 +h2 zµ∂z ∂v¶2 . (6.7) Of course for cartesian coordinates x, y, z we have hx=hy=hz= 1. This gives the explicit formula for the metric on the surface in terms of the equations giving x, y, z in terms of u, vthat define the surface. Often one writes the result for the length of a curve in the surface in the form s=Z Cp E du2+ 2F du dv +G dv2. (6.8) This just means that one can use any convenient parameter. 6.2 Area The formula for the area of a surface is A=Z Sarea =Z S√g du∧dv=Z Sp EG−F2du∧dv. (6.9) Here g=EG−F2is the determinant of the metric tensor. This is particularly simple in the case of orthogonal coordinates, in which case F= 0. As an example, take the surface given in cartesian coordinates by z=x2+y2 with z≤1. There are various parameterizations. Take, for instance, x= rcos(φ),y=rsin(φ),z=r2. Then E= 1 + 4 r2,F= 0, and G=r2. So with these parameters the area form is√g dr∧dφ=r√ 1 + 4 r2dr∧dφ. The area is 2πtimes the integral from 0 to 1 of r√ 1 + 4 r2dr. The area is thus ( π/6)(53 2−1). Again, take the surface z=x2+y2with z≤1. This time use x, yas parameters. Then E= 1 + 4 x2,F= 4xy, and G= 1 + 4 y2. So with these parameters the area form is√g dx∧dy=p 1 + 4 x2+ 4y2dx∧dy. This is integrated over the region x2+y2= 1. The answer is of course the same. There is an alternative expression for the surface area of a surface inside Euclidean space that is sometimes convenient. This is A=Z Sarea =Z Ss h2yh2zµdy∧dz du∧dv¶2 +h2zh2xµdz∧dx du∧dv¶2 +h2xh2yµdx∧dy du∧dv¶2 du∧dv. (6.10) Again for cartesian coordinates x, y, z we have hx=hy=hz= 1. Here a fraction such as dy∧dzdivided by du∧dvis a ratio of 2-forms on the surface S. As we know, such a ratio is just a Jacobian determinant of y, zwith respect tou, v. Once again, take the surface given in cartesian coordinates by z=x2+y2 with z≤1. Use x, yas parameters. Then ( dy∧dz)/(dx∧dy) =−2xand (dz∧dx)/(dx∧dy) =−2yand ( dx∧dy)/(dx∧dy) = 1. So the area form is√g dx∧dy=p 4x2+ 4y2+ 1dx∧dyas before. 6.3. DIVERGENCE AND STOKES THEOREMS 35 6.3 Divergence and Stokes theorems This section has a discussion of the form that the divergence and Stokes’s the- orems have in the context of orthogonal coordinates. The first topic is the divergence theorem in two dimensions. Say that the vector field vhas an expression in terms of normalized basis vectors of the form v=a1 hu∂ ∂u+b1 hv∂ ∂v. (6.11) Recall that the area form is area = huhvdu dv. (6.12) Then the corresponding differential 2-form is v⌋area = ahvdv−bhudu. (6.13) The divergence theorem in two dimensions is obtained by applying Greens’s theorem for 1-forms to this particular 1-form. The result is Z R·1 huhvµ∂ ∂u(hva) +∂ ∂v(hub)¶¸ huhvdu dv =Z ∂Rahvdv−bhudu. (6.14) The expression in brackets on the left is the divergence of the vector field. On the right the integrand measures the amount of the vector field crossing normal to the curve. The next topic is the divergence theorem in three dimensions. Say that the vector field vhas an expression in terms of normalized basis vectors of the form v=a1 hu∂ ∂u+b1 hv∂ ∂v+c1 hw∂ ∂w. (6.15) Recall that the volume form is vol = huhvhwdu dv dw. (6.16) Then the corresponding differential 2-form is v⌋vol = ahvhwdv dw +bhwhudw du +chuhvdu dv. (6.17) The divergence theorem in three dimensions is obtained by applying Gauss’s theorem for 2-forms to this particular 2-form. The result is Z V·1 huhvhwµ∂ ∂u(hvhwa) +∂ ∂v(hwhub) +∂ ∂w(huhvc)¶¸ huhvhwdu dv dw = Z ∂Vahvhwdv dw +bhwhudw du +chuhvdu dv. (6.18) The expression in brackets is the divergence of the vector field. 36 CHAPTER 6. LENGTH AND AREA The divergence theorem works the same way in ndimensions. A vector field gives rise to an n−1 form whose differential is an nform. This nform is a scalar times the volume form. The next topic is the classical Stokes’s theorem in three dimensions. Say that the vector field vhas an expression in terms of normalized basis vectors of the form v=a1 hu∂ ∂u+b1 hv∂ ∂v+c1 hw∂ ∂w. (6.19) Thus the vector field is expressed in terms of normalized basis vectors. Then there is a corresponding differential 1-form gv=ahudu+bhvdv+chwdw. (6.20) The classical Stokes’s theorem in three dimensions is obtained by applying Stokes’s theorem for 1-forms to this particular 1-form. This gives on the left hand side Z S·1 hvhwµ∂hwc ∂v−∂hvb ∂w¶¸ hvhwdv dw +·1 huhwµ∂hua ∂w−∂hwc ∂u¶¸ hwhudw du + ·1 huhvµ∂hvb ∂u−∂hua ∂v¶¸ huhvdu dv (6.21) and on the right hand side Z ∂Sahudu+bhvdv+chwdw. (6.22) The terms in square brackets are the components of the curl of the vector field expressed in terms of normalized basis vectors. Notice that the fact that the curl may be expressed as a vector field depends on three dimensions. In general a vector field gives rise to a 1-form whose differential is a 2-form. In two dimensions this 2-form may be identified as a scalar times the area, while in three dimensions this 2-form may be associated with a vector field. The situation with the cross product is similar. Two vectors give two 1- forms, whose exterior product is a 2-form. 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