colombeau distribution theory

BULLETIN (New Series) OF THE AMERICAN MATHEMATICAL SOCIETY Volume 23, Number 2, October 1990 MULTIPLICATION OF DISTRIBUTIONS J. F. COLOMBEAU Physics often puts in evidence products which look meaningless in a mathematical sense and appear under the form of "heuris­ tic multiplication of distributions." The most famous example is probably quantum electrodynamics founded in 1927; it was soon recognized that it led to "infinite quantities" in the form of di­ vergent integrals from which much later (1947) finite predictions were extracted, see [4] for instance. Such products appear also in elasticity and elastoplasticity (shock waves; their importance is presently emphasized in elasticity and elastoplasticity by the need for numerical simulations of collisions), in acoustics (sound prop­ agation in a medium with discontinuous characteristics), and in other domains, see [1-3, 10, 13, 14, 16]. These examples provide motivation for mathematical attempts to define and study the multiplication of distributions. The prob­ lem is difficult since L. Schwartz (1954) proved the impossibility of a straightforward extension of the product of continuous func­ tions, see [8, 26]. Strictly speaking, the reader of this text does not need to know anything about distributions in order to follow our discussion. If Q is a nonvoid open set of the space R" we denote by ^°°(Q) or by 3f(Çl) the space of all ^°° functions on Q which are null outside of a variable compact subset of Q ; such functions exist: to provide an example consider the auxiliary function of one real variable f(x) = exp(l/(x2 - 1)) if |JC| < 1, f(x) = 0 if \x\ > 1. ^°°(Q) is endowed with a topology and the distributions on Q are defined as the continuous linear maps from ^°°(Q) into C. Any locally integrable function ƒ on Q defines a distribution, V-> f(x)tp(x)dx, 0>€g^°(Q). JQ Received by the editors October 12, 1988. 1980 Mathematics Subject Classification (1985 Revision). Primary 46F10, 35D05, 35D10. ©1990 American Mathematical Society 0273-0979/90 $1.00+ $.25 per page 251 252 J. F. COLOMBEAU If T is a distribution on Q, then its partial derivative dT/dx{, 1 < i < n9 is defined by dT/dxx{(p) = -T{d(pldxt) which is nothing else than a formal integration by parts formula. The con­ cept of distribution provides a convenient setting in which one can freely differentiate functions which are not derivable in the classi­ cal sense. In the sequel we shall limit ourselves to the case Q, = R ; this is done to simplify the notation. 1. THE ORIGIN OF OUR CONCEPT OF GENERALIZED FUNCTIONS The aim of this section is to sketch the original idea which led us to define a general multiplication of distributions. For this we use the nonelementary concept of differentiable functions defined on an infinite-dimensional vector space. Since we shall only sketch the idea the reader does not need to know this concept [6]; some analogy with the case of differentiable functions defined on R" is enough. We denote by I? or W°° the space of all C°° functions on R (with a natural topology), by I?' the space of all linear continuous maps from I? into C and by 2 the space 2(R). I? and 2 are infinite-dimensional vector spaces and have natural topologies. We denote by W00^') and ^°°{2) the respective spaces of all C°° functions on &' and 2 . In distribution theory one proves that any element of I?' can be approximated (in the topology of I?' ) by a sequence of elements of 2 ; i.e. 2 is a dense subspace of %'. This implies that ^00(ê?f) is contained in W°°{2) through the map (p -+ <p^ if <p e <^00{^') and if g>^ is its restriction to 2. Let Sx denote the Dirac measure at the point x G R, i.e. Sx{cp) = cp(x), cp e 2 . If 9JI is the map gr700^') _ g?00 tp x^ (p(ôx) then one shows easily that the algebras W°° and ^(If^/Keratt are isomorphic. The conclusion is that certain quotients of spaces of C°° functions over certain locally convex spaces can be inter­ preted as very nice algebras of functions. An idea is: extend Ker DJl to an ideal JIT of W°°(2) (or of a subalgebra sf of W°°(2) as large as possible) and try to interpret the elements of the quotient algebra as "generalized functions." This approach has been developed in [7, 8]. Fortunately it soon became clear that one can drop the MULTIPLICATION OF DISTRIBUTIONS 253 sophisticated concept of C functions over 31. This gives a very elementary construction which we expose in Appendix 1. One can go on reading this text just by keeping in mind that the elements of & have essentially the main properties of the C°° functions. We have the inclusions ^°° is a subalgebra; & induces on 3f' the partial derivatives in the sense of distributions. 2. COHERENCE WITH THE CLASSICAL MULTIPLICATION From Schwartz's impossibility result it follows that the algebra fê of all continuous functions on R cannot be a subalgebra of &. The following simple calculation shows that the algebra 8y of all piecewise continuous functions on R is not a subalgebra of 9. Let Y e &f be the Heaviside function (Y(x) = 0 if x < 0, Y(x) = 1 if x > 0). In the algebra ^ one has if n = 2, 3, ... (1) Yn = Y. Thus by differentiation (2) Yn~XY' = -Y'. n Multiplication by Y gives n Use of (2) gives (3) -J—Yf = ^-Yf v J n + 1 In which is absurd ( Y1 is the Dirac delta mass). The study of a shock wave with an elastic-plastic phase transition shows that in physics one needs several different Heaviside functions: they are identical to 0 for x < 0, to 1 for x > 0 and differ by their "microscopic" behavior at the point 0, see [3, 10, 14]. Therefore we interpret the absurd result (3) as a consequence of (1), which should be replaced by (1') Yn^Y iîn^X. The classical product in g^, i.e. (1), cannot be used for calcula­ tions involving multiplications of distributions. Indeed in & one has (l'). The algebra 8^ is not a subalgebra of &. Schwartz's 254 J. F. COLOMBEAU result has been interpreted negatively; for us the same fact is in­ terpreted positively: since physics imposes the need for several different Heaviside functions it is very fortunate that mathematics leads to the same conclusion. It is fortunate also that the classical product in 9^ is very close to the new product in «^ : in order to formulate this we introduce the concept of "association": an element G of & is said to be associated with 0 iff for any y/ e ^°°(Q) the integral / G(x)i//(x)dx JR is null in a natural sense, see Appendix 1. We say that G{ and G2 e & are associated with each other iff G{ - G2 is associated with 0 ; we write G{ « G2. One proves easily that [7, 8]: Proposition 1. Two distributions are associated iff they are equal Proposition 2. The classical product of piecewise continuous func­ tions (when considered as an element of & through the inclusion 2y c & ) is associated with their product in &. A similar result holds for most classical multiplications of dis­ tributions, see [3, 8, 22, 26]. Thus the association is an extension of the equality of distributions and through it one obtains coherence of the new product in & with the classical products when the latter exist, 3. "SAFETY BARRIERS," OR REGULARITY RESULTS In order to manipulate freely one must leave the ordinary world of classical functions. Then one finds "abstract objects" that are solutions of equations. The genuine difficulty is shifted to the fi­ nal task of ascertaining whether the solutions thus obtained are indeed "classical objects" which are capable of representing phys­ ical quantities. This has been done in the context of the present theory [3, 5, 23-25]. Note that for certain equations which have sufficiently many classical solutions the abstract solutions are auto­ matically classical solutions; these results are welcome and beau­ tiful from the mathematical viewpoint. They are all the more useful in the present setting as there are "wild objects" in 9 : for instance if ô is the Dirac distribution at the origin the "pointval- ues" ô(0), ô2(0), ... can be considered as constant generalized MULTIPLICATION OF DISTRIBUTIONS 255 functions (one defines pointvalues of elements of 9 as "general­ ized numbers" by following, for fixed x, the pattern of the defi­ nition of & ). Their classes in & are solutions of G' = 0. They are eliminated at once by classical initial or boundary conditions (which hold in a physical context); one proves easily Proposition 3. Let G G 9. Assume G' = 0 and assume there is x0 G R such that G(x0) is a classical number. Then G is identical to this constant. If G' « 0 one obtains a similar result, see [3]. In many nonlinear problems these wild constants are eliminated even without initial or boundary conditions [21]: Theorem 1. Let P(x, y) be a nonzero polynomial in two variables. Let I be an open interval. Then if G e S? (I), P(x, G) = 0 if and only if G is a classical C°° solution on I. Direct proofs of Corollaries 1, 2 and 3 are easy and given in Appendix 2. Corollary 1. If P is a nonzero polynomial in one variable, then P(G) = 0 if and only if G is identical to a classical root of P. Corollary 2. The equation xG = 0, G e &, implies G = 0 on the whole of R (i.e. the singularity x = 0 does not allow new solutions). Corollary 3. The equation G2 = x2, G e &, implies G equals one of the two C°° solutions +x or -x (the classical solutions \x\ and -\x\ are excluded). The equation G — x has no solution on the whole of R. Note that the classical solutions ô, ±\x\ and combinations of ±\/W are recovered with the association (as solutions of xy « 0, y - x « 0 and y — x « 0, respectively). From Proposition 2: Proposition 4. The classical continuous solutions are recovered if one states the equation in the association sense, i.e. P(x, G) « 0. Algebraic differential equations (ADE) provide a setting in which multiplication and derivation are combined. An ADE is an equation of the form (4) P(x,y(x),y'(x),...,y{m)(x)) = 0 where P is a nonzero polynomial in m + 2 variables. 256 J. F. COLOMBEAU Problems. Let G e & and let us assume there is x0 E R such that the pointvalues G(xQ), G'(x0), ... , G(m_1)(x0) are classical numbers. If (4) holds in 3? with y = G, in which conditions is G a classical solution? (See a counterexample and Corollary 4 in Appendix 2.) Is it possible to drop the condition at x0 if G is in the algebra spanned by the distributions? The study of the nature of different kinds of solutions of ADEs in conjunction with the results exposed in [27-29] would clarify the "standard" or "nonstandard" character of these contexts; perhaps it could also clarify the concept of solution of an ADE; note that if y is a function of class Cm then P(x, y(x), ... , y{m)(x)) « 0 in & if and only if y is a solution in the classical sense. 4. GENERALIZED SOLUTIONS OF PARTIAL DIFFERENTIAL EQUATIONS I: MATHEMATICS For large classes of equations which do not have solutions within distribution theory one can obtain existence-uniqueness results in the present setting. These new solutions are always associated with the classical solutions when the latter exist. We only give one typical example [23]. Consider the initial value problem for the semilinear hyperbolic system in two variables (dt + A(x, t)dx)u(x, t) = F(x,t9u(x, t)) (x, t) e R2 u(x, 0) = uQ(x) x e Rn u : R2 —• R" , A(x, t) is a smooth real-valued diagonal n x n matrix such that A or dxA is globally bounded; the function F : R xR"-^R" is smooth and satisfies the bounded gradient con­ dition: for any compact subset K of Rn and any j = 1, ... , n (6) sup \d F(x, t, u)\ < +oo. (x, t)eK J u£Rn Further, in order to define F(x, t, u(x, t)) for any u e (^(R2))" let us assume that the map u —• F(x, t,u), together with all derivatives, is polynomially bounded, uniformly for (x, t) in compact subsets of R (remark in Appendix 1). Theorem 2. For any given u0 e (&(R))n, system (5) has a unique global solution u e (^(R2))" . Moreover if u0 e (L^R))" then MULTIPLICATION OF DISTRIBUTIONS 257 this solution M G (^(R ))" is associated with the classical Lloc(R ) solution. Here L\OC denotes as usual the classical space of locally inte­ grate functions for the Lebesgue measure; such a global solution is known to exist in this setting. One can perform explicit computations, for instance, in the case where u is a distribution with support at finitely many points, see [23]. Other examples are given in [3, 5, 9, 12, 24-26]. 5. GENERALIZED SOLUTIONS OF PARTIAL DIFFERENTIAL EQUATIONS II: PHYSICS In many domains the equations of physics put in evidence "mul­ tiplications of distributions" which follow directly from the state­ ment of constitutive equations. In a few cases it appears impos­ sible to avoid this fact and one really needs to deal with it. For instance Hooke's law of elasticity is a linear stress-strain relation­ ship [20]; but in strong collisions (such as those occurring between projectiles and armour) there is not even a bijective relationship between stress and strain. In this case most physicists and engi­ neers state Hooke's law in infinitesimal form in a frame of ref­ erence following the medium; in elastoplasticity this unavoidably gives rise to multiplications of distributions in the case of shock waves, see [3, 10, 13, 14, 16]. The following is a simplified model of elasticity in a one-dimen­ sional homogeneous medium (7) (pu)t + (pu2)x = ax, 2 °t + uax = k ux> where p = density, u = velocity, a = stress, k > 0 is a con­ stant depending on the medium and obtained from experiments (Hooke's law). Often the term uax is dropped in the literature (linear approximation) but for numerical simulations of collisions it plays a basic role. Numerical codes of engineers have put in evidence "solutions" (p, u, a) of (7) which are discontinuous on the same curve in the (x, t) space; they represent shock waves which have been observed by physicists on the occasion of colli­ sions. Then the term uax appears in the form of a product of a 258 J. F. COLOMBEAU discontinuous function and a derivative of a discontinuous func­ tion, whose singularities overlap: this product does not make sense within distribution theory. Thus one is led to formulate (7) in the present setting. One can prove that (7) has no discontinuous solution if all equations in (7) are stated with the (strong) equality in &. On the other hand one can prove that, if one states all equations in (7) with the associ­ ation, then (7) admits an infinite number of different solutions, depending on an arbitrary real parameter (arising from the term uox ) (see Appendix 3). This ambiguity only shows that this weak formulation does not contain enough physical information. One resolves naturally the ambiguity as follows: The two first equations in (7) express the basic physical laws of mass and momentum conservation while the third one is a consti­ tutive equation depending on the material and on the conditions of the experiment. A natural way to state (7) is: (7') (pu)t + (pu2)x = (Tx, ot + uax &k ux. The association there expresses that, in the very small width of the shock (several times the average distance between molecules) the constitutive equation is no longer valid while mass and momentum conservation are valid there. One proves that (7') has discontin­ uous solutions and nonambiguous jump conditions on the shocks; see [3, 14] and Appendix 3. Our setting gives formulas, numer­ ical methods and justifies existing numerical codes elaborated by engineers. Thus it gives the possibility to investigate problems of physics which could not be attacked mathematically within distri­ bution theory. This method has been successfully applied to more complicated systems of elasticity, elastoplasticity, and acoustics, see [1-3, 10, 11, 14, 16-18]. Thus our new concepts have permitted us to predict numerical results that emerge from experiments. In some cases it was previ­ ously unknown how to obtain them. When some data are available the results of our calculations agree qualitatively and quantitatively with the expected results [3, 10, 11, 13-16, 18]. Let us pause for a minute on this basic achievement, so as to understand its mechanism. The ambiguities appearing in equations of physics, when these equations involve "heuristic multiplications of distri- MULTIPLICATION OF DISTRIBUTIONS 259 butions," correspond to the fact that these equations, when stated in weak form (i.e. with association), have an infinite number of so­ lutions. This point was essentially known and understood without our theory (consider in quantum field theory the Hahn-Banach method of Bogoliubov-Parasiuk [4]). The basic point is that our new setting has suggested more precise formulations of the equa­ tions, on physical ground, in which there is no more ambiguity. To resolve the ambiguity physics and mathematics have been used conjointly and simultaneously, each of them playing its natural role. 6. GENERALIZED SOLUTIONS OF PARTIAL DIFFERENTIAL EQUATIONS III: SYSTEMS OF CONSERVATION LAWS A conservation law is an equation in divergence form (8) M, + Div(/(K)) = 0. Many physical laws are conservation laws; the more important system of conservation laws is certainly the system of fluid dynam­ ics; in one dimension and absence of viscosity, thermal effects and external forces it is the system of equations Pt + (PU)X = °> (9) (pu)t + (pu2+p)x = 0, (pe)t + (peu+pu)x = 0, where p = density, u = velocity, p = pressure, and e = density of total energy. Since (9) is a system of three equations with four unknowns it is complemented by a constitutive equation (9') p = <t>(p,e-±u2). All these equations are understood in the sense of distribution the­ ory in the case of shock waves. According to the method exposed in the above section we state (9), (9') in the more precise form Pt + (PU)X = °> ,9//v (pu)t + {pu2+p)x = 0, (pe)t + (peu+pu)x = 0, p « 0(/?, e - \u ). It can be easily shown that (9") has travelling wave solutions (i.e. solutions which remain constant on both sides of the discontinuity 260 J. F. COLOMBEAU and propagate with constant speed) and that they are solutions of the system of equations in nonconservation form vt + uvx -vux = 0, (10) ut + uux + vpx = 09 yv)l%^V ' p)) (/?' + UPx) + \P + %{V >p)) Ux * °' where v = l/p is the specific volume and where the constitutive equation has been stated in the form e - \u2 « <p(v, p). Note that (10) has no discontinuous solution in the sense of distribution theory, but has discontinuous solutions in our setting. Travelling wave solutions of (10) have a very simple form [3, 14] and, from this study, one can build numerical schemes for the solution of (10) [3, 13, 15, 18]. In certain circumstances this method is very efficient (modelling of the behavior of solids submitted to strong constraints). Note also that solutions in & of systems of conservation laws are very closely connected to the measure valued solutions of DiPerna [19]. 7. CONCLUSION In conclusion one can stress the basic role played by the dissoci­ ation of the classical concept of equality into the strong equality in 9 and into the weak equality «. This dissociation permits one to circumvent Schwartz's impossibility result, and so to obtain a general multiplication of distributions (enjoying all computational properties) coherent with classical analysis. From a viewpoint of applied mathematics and physics it is at the very basis of the res­ olution of ambiguities and thus of the predictions of results that could be checked from experiments. A posteriori one can real­ ize that this dissociation had already been perceived, in the very classical setting of ADEs by Rubel [27-29] in his dissociation of solutions of ADEs into C°° ones and "pointwise" ones (i.e. only differentiable enough to plug into the ADE). We can also retain that the theory presented in this paper can be considered as some kind of "nonstandard analysis" since it realizes a calculus involv­ ing "infinitesimal quantities." There the concept of "shadow" of a nonstandard function might play the role of our association. An up to date set of references can be found in the second edition of [3]. MULTIPLICATION OF DISTRIBUTIONS 261 APPENDIX 1. DEFINITION OF GENERALIZED FUNCTIONS If q = 0, 1, 2, ... we set stfq — \(p e 31 such that / (p{X) dk = 1 and /+oo ï A>(A)rfA = 0if 1 <i<q\. One proves easily that srfq is nonvoid; if cp G 3f and e > 0 we set Then it is immediate that tp G stfq if and only if cpe e stff . We denote by ^ the set of all functions F: sf0 xR->R <p,x F(cp,x) such that for every ^ JGJ/0 the map Rx]0, l[xR^R 6 e x F([dyt + {\-0)x\B9x) is C°° in the variables (0, e, x). We denote by g^ (where the subscript M stands for moderate) the subset of <8^ of all functions F such that Vw, m G N 37V G N such that if p E J^ there are (*) c, r\ > 0 such that sup dx' îF{(pE,x) < ce if 0 < e < >/. The symbol (*) always means that c and r\ can be chosen inde­ pendent of cp when (p ranges in a closed line segment in the set s/N (i.e. <p = dv + (l-0)x, V, xes/N,0<6< 1). Clearly %M is a subalgebra of <8^ (for pointwise multiplication). We denote by JV the set of all functions F e ë?0 such that Vn, m G N Vp E N 3q e N such that if 9? G J^ there are (*) c, Y] > 0 such that sup dx' îF(VP,x) <cep if 0 < e < f/. Clearly A* is an ideal of WM and so the quotient space 262 J. F. COLOMBEAU is an algebra. It is proved that the elements of & have essentially the properties of C°° functions (they are local objects, derivation, multiplication,... ) see [3, 9, 26]. Remark. The bounds defining JV came from the idea of extension of the ideal Ker9Jt of ^°°(r'); then the bounds defining %M came from the idea of finding a subalgebra of <^>00(^) for which JV would be an ideal, see [7, 8]. To g G £?°° we associate Re%M defined by R(q>, x) = g(x). One shows easily that this defines an inclusion ^°° c *&. Let us denote by £y the set of all piecewise continuous func­ tions on R (i.e. continuous except on a discrete set, on which they have right and left limits). To g € 8} we associate Reê?0 defined by /+oo g(X)(p(X- x)dk. -oo One proves at once that Re %M and that this defines an inclusion fêr c &. If g e &00 one checks at once that the choice between the two above-mentioned functions R is insignificant modulo Jf. If T is a distribution one defines R by an immediate extension of the above formula and one obtains an inclusion of the set 3t' of all distributions into 3f. If one does not know the distributions one may define them as those elements of 9 which are, in the neighborhood of each real number, some derivative of a continu­ ous function. Remarks. In [7, 8] we gave a slightly different definition of the ideal Jf ; it has been recognized subsequently [9] that the defini­ tion given above is necessary to provide uniqueness of asymptotic expansions, uniqueness of analytic continuation, and also Theo­ rem 1 below. In [9] the differentiability of F in 6 and e, which is present in [7, 8], was dropped for simplification. It was recognized later that this condition is needed to obtain Theorem 1. More generally than multiplication, one can define ƒ((?), G e &(Rn) arbitrary, provided that ƒ is a C°° function which, to­ gether with all its derivatives, is polynomially bounded. The concept of equality in 9 is very strict. We introduce the weaker concept of "association": an element G of 9 is said to be associated with 0 iff for any y/ G ^°°(Q) the integral / R(q>e, x)y/(x)dx JR MULTIPLICATION OF DISTRIBUTIONS 263 tends to 0 when e —• 0 ( R is a representative of G, (p is in stfN for large enough N ). We say that Gt and G2e& are associated with each other iff Gx - G2 is associated with 0; we write Gx « G2 . The concept of association is a faithful generalization of the classical concept of equality of integrable functions and distributions. But it is through our concept of equality in 9 that we are able to define the product of distributions. APPENDIX 2. PROOFS OF REGULARITY RESULTS Direct proof of Corollary 1. Let a{, a2, ... , an be the classical roots of P, at ^ a. if i / j. By assumption we have a bound \P(R(<pe ,x))\< cqea{q) if R is a representative of G ( a(#) tends to +oo when q tends to +oo ). Taking into account the multi­ plicity of the roots of P we get (11) \R(<Pe > •*) - 0,- I < c'q£a for some other c , a , where a. depends a prion' on e, q> and x; but from the con- tinuity of RUpp, x) in e, © and x one has a. independent of £, #> and x. It remains necessary to obtain a bound for the x- derivatives of R(<pe, x). One has ^P(R(<Pe, x)) = P'(i?(?>e, x)) • R'(cpe, x) so that if /''(a,- ) ^ 0 one has at once the required bound for \R'(<pp, JC)| . If P\a. ) = 0 and if Pn(a. ) £ 0, the formula O tri »n d 2 .2 />(*(?, ,*)) = P"(R((pe, x)). (R'(ç>e, x)) 2 + Pf(R((pe,x))-R"((pe,x) gives at once the required bound for \R'(ç>e, JC)| using (11) to get rid of the last term. By induction on the order of multiplicity of a. , we get a bound \R'(<pF, x\ < c"ea ^ . Similarly one gets bounds for higher-order derivatives of R. Direct proof of Corollary 2. xy = 0 =» je/ + y = 0 => x2/ = 0 =» x2(y')3 = 0. Let ü(pe, x) be a representative of y. In abbre­ viated notation we have \x2R,3(<pe, x)\ < cqea ^ . Since |JC|"~ ' is integrable at 0 one finds that \R(<pe9 x)\ < c'sa ^ for some 264 J. F. COLOMBEAU other c, a (if x > 0 use the formula R(cpe, x) = i?(#?e, 1) + f*R'(<pe, k)dk\ if x < 0, replace 1 by -1 ). To get a similar bound for R'(<pe, x) start from x y = 0; setting z = y' one obtains as at the beginning of this proof that x z'4 = 0. One con­ cludes from the fact that \x\~ /4 is integrable at 0. x2y' = 0 => 2xy' + x/' = 0^ x3/' = 0, from which one can use the same method to get the desired bound for Rf'. And so on. Direct proof of Corollary 3. y = x in £?(R) implies y y =x, y2y' = yx and so x2y' = xy , i.e. x(xyf -y) = 0. From Corollary 3, y = xy'. This equality and (y - x)(y + x) = 0 imply x2(y' - 1) x (j/ + 1) = 0. Corollary 3 gives (j/ - l)(y' + 1) = 0. Corollary 2 gives that y is identical to the constant +1 or to the constant -1. Integration (Proposition 3) gives y - x + cx or y = -x + c2 , 2 2 where q , c2 are generalized numbers. From y = x one gets cx = 0 = c2. Another consequence of Theorem 1 is: Corollary 4. Under the conditions on P and I in Theorem 1 let us consider the ADE P(x, y^m\x)) = 0 in S?, completed by the condition y(x0) = a0,...,y (*o) = a«-i> a( € R, 1 < / < m - 1 #«6? x0 e ƒ. r/?e« P(x, G(m~l)) = 0 w 5?(7) #G is a classical C°° JO/MÖO/I. Proof. Apply Theorem 1 with G ' and integrate (Proposition 3). Counterexample. The class of ü(p, x) = f(ç>)exp(l/(x - 1)) if -1 < x < 1 and R(ç>, x) = 0 if x < -1 or i > 1, where f{(P) = S(P (k)dk9 is solution of the equation 2xy+{x - \)y' = 0. APPENDIX 3. EXAMPLES OF SHOCK WAVE CALCULATIONS We sketch how one can compute shock wave solutions of system (7'). First consider the weak formulation Pt + (pu)x&0, (7") (pu)t + (pu2)x^<jx, °t + uax ^ k ux - MULTIPLICATION OF DISTRIBUTIONS 265 We seek travelling wave solutions of the form p(x, t) = ApH(x - vt) + px, (12) u(x, t) = AuK(x - vt) + ux, a(x, /) = AoL(x - vt) + ax, where v (= velocity of the shock), Aw = wr - w{, w (w = p, u, a) are real numbers and where H, K, L are three possibly different Heaviside generalized functions (i.e. they are associated with the classical Heaviside function). Putting (12) into (7/;) one obtains at once the three jump formulas (v - ux)Ap = (Ap + p{)Au, (13) (v - u{ - Au)(ApAu + pxAu + u{Ap) = u{p{Au - Aa, (v — u{)Aa = AA uAo — k Au, where the real parameter A is defined by Kl! « AS . There is an infinite number of possible jump conditions depending on the real parameter A. Now let us consider the stronger formulation (?). The first equation in (?) gives / A ^N iV Au T^/ rr Au _../ _ (-v + u{ + AuK)H + /^ — AT // + /?! -£-K = 0 from which one obtains easily H as a function of AT by solving as usual the differential equation a(x)y' + b(x)y + c(x) = 0 in ^. The second equation in (7') gives L as a function of ƒƒ and #. Finally one obtains L^K and thus KL! = ATAT' » ^; (by differentiation of K2 « AT), i.e. A = \. (?') has nonambiguous jump conditions. In general one finds values of numbers like A which are different from j . Various examples of this kind of calculations are given in [3, 10, 11, 14, 16, 18]. APPENDIX 4. DISTRIBUTIONS IN MATHEMATICS AND IN PHYSICS We have a canonical inclusion 3t' c & and, at the same time, one is forced in physical applications to consider several Heaviside like, Dirac like,... functions. There is no paradox if one thinks about the different ways mathematicians and physicists conceive and use distributions. For mathematicians the space 2' is defined modulo an iso­ morphism (concerning all operations). It is such an isomorphic copy of 3J' which is canonically included into &. If permits, via 266 J. F. COLOMBEAU the multiplication in *§ and the association, a synthesis of most existing multiplications of distributions. For physicists the space «Sr/ is considered as a reservoir of math­ ematical objects used to describe the physical world. In our context the use of the above subspace 2' of & as such a reservoir may lead to mistakes in some cases involving "multiplications of dis­ tributions." Then the correct reservoir is 9 itself, which contains several Heaviside like, Dirac like,... functions. In this way a nonambiguous mathematical multiplication of distributions can be reconciled with the well-known fact that in physics "multiplications of distributions" such as Yô or ô , can give different results according to the context. Distributions origi­ nating in physics have to be represented by various elements of &, usually not those in the subspace 3f' of &. This reminds one of the nonbijective correspondence between quasi-standard functions (a subclass of the nonstandard functions) and distributions. This suggests that our theory could be considered as some refined (with respect to certain properties) version of nonstandard analysis. ACKNOWLEDGMENTS The author is very much indebted to an unknown referee for suggesting an overall change in the text, and to Dr. J. T. Donohue for corrections and clarifications. REFERENCES 1. M. Adamczewski, J. F. Colombeau, and A. Y. Le Roux, Convergence of numerical schemes involving powers of the Dirac delta function, J. Math. Anal. Appl. 195 (1) 1990, 172-185. 2. A. Y. Barka, J. F. Colombeau, and B. Perrot, A numerical modeling of the fluid/fluid acoustic dioptra, J. Acoustique (in press), 1989. 3. H. A. 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