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try to show the two tensorized forms are the same v3
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Working notes by Phil dated 1.11.15, part of the May 2015 update to his curvilinear-systems tensor document. He tries to prove that the tensorized forms (15.7.9) and (15.2.8) of the vector Laplacian of B agree, using epsilon-tensor contraction identities and metric manipulations. The algebra does not close, so he examines his claim that tensorization is unique and finds that (B^j;j);n and B^j;n;j differ. He then re-reads his proof near (15.2.5).
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Show two tensorized forms are same v 3 PhL 1.11.15
Here are the "two forms" for (B)'n
(B)'n = (B'j;j);n – g'-1/2ε'nab(g'-1/2g'bcε'cdeB'e;d);a (15.7.9)
(B)n = Bn;j;j . (15.2.8)
We want to show these are the same. Remove all primes and we want to show that
(B)n = (Bj;j);n – g-1/2εnab(g-1/2gbcεcdeBe;d);a (15.7.19)
(B)n = Bn;j;j . (15.2.8)
I can simplify the first line using what I now know so it reads,
(B)n = (Bj;j);n – g-1/2εnab(g-1/2gbcεcdeBe;d);a
= (Bj;j);n – g-1/2εnabg-1/2εcdegbc Be;d;a !!
= (Bj;j);n – g-1εnab εcdegbc Be;d;a
= (Bj;j);n – g-1εnab εbde Be;d;a
So our entire task is reduced to showing this fact
Bn;j;j = Bj;j;n – g-1εnab εbde Be;d;a ?
We would like to get all the B indices to the same level, so let's choose B*** so start with
Bn;j;j = Bj;j;n – g-1εnab εbde Be;d;a ?
which I choose to rewrite as
Bn;a;a = Be;e;n – g-1εnab εbde Be;d;a ?
Now we have three different "tilts" to worry about on the B indices. Let's process the first one
Bn;a;a = δneBe;a;a = δneδad Be;d;a
Now process the second term
Be;e;n = gedBe;d;n = gedgnaBe;da
The problem is then to prove that this is true
δneδad Be;d;a = ged gna Be;d;a – g-1εnab εbde Be;d;a ?
or
[ δneδad – ged gna + g-1εnab εbde ] Be;d;a = 0 ?
or
[ gnegad – ged gna + g-1εnab εbde ] Be;d;a = 0 ?
This will be proved if we can show that
gnegad – ged gna + g-1εnab εbde = 0 ?
or
gnegad – ged gna + g-1εbna εbde = 0 ? εnab = εbna
I don't think this is true, so we have a problem. At least it is a tensor equation in terms of index positions. Here is something I think IS true from above (D.11.10)
|g|-1 εcabεca'b' = δaa'δbb' – δab'δba' = gaa'gbb' – gab'gba'
Ignoring the abs value for now, this says
g-1 εcabεca'b' = gaa'gbb' – gab'gba'
or
g-1 εbnaεbde = gdngea – gdagen
I think this is a true equation. Then I have to show this
gnegad – ged gna + gdngea – gdagen = 0
or
– ged gna + gdngea = 0
or
ged gna = gdngea
or
ged gna = δdnδea
But I just don't see how this can be true!! Is this some Fact that I have missed? I do know
g** = (g**)-1 = cof(g**)/det(g**) = cof(g**)/ g
So this says
gna = cof(gna)/ g
and then
ged gna = ged cof(gna)/ g
Suppose the coordinate system is orthogonal! Then consider
ged gna = he2δed hn-2δna
= (he2hn-2) δedδna
but this is a far cry from what I need to be true.
Back up and ponder uniqueness of tensorization
Consider these two paths:
(∂j2)Bn = ∂j∂j Bn = Bn,j,j → Bn;j;j
(∂j2)Bn = ∂j∂j Bn = Bn,j,j → Bn;j;j
My claim that "tensorization is unique" would then imply that
Bn;j;j = Bn;j;j
which seems reasonable from my reverse-tilt-rule contraction theorem.
But consider
Bn;j;j = Bn;a;a = gab Bn;b;a = gabgac Bn;b;c = δbc Bn;b;c = Bn;b;b = Bn;jb;j QED
So what is my problem?
Try a different case:
∂n(∂jBj) = (Bj,j),n → (Bj;j);n
∂j(∂nBj) = Bj,n,j → Bj;n;j
My claim that "tensorization is unique" would then imply that
(Bj;j);n = Bj;n;j ? (*)
This is no longer a simple case of the reverse tilt rule. And this claim is NOT TRUE as I will now show.
First, process the left side of (*)
(Bj;j);n = Σj(Bj;j);n = Σj(ΣiδjiBi;j);n = Σij δji(Bi;j;n)
= Σij δji(Σm gjmBi;m;n) = Σijmδjigjm (Bi;m;n) = Σijmδjigjm (ΣsgnsBi;ms)
= Σijms δjigjmgns (Bi;ms) = Σjms gjmgns (Bj;ms)
Now process the right side of (*)
ΣjBj;n;j = Σjmsδnm δjs Bj;m;s
then (*) becomes
Σjms gjmgns (Bj;ms) = Σjmsδnm δjs Bj;m;s ?
or
Σjms[ gjmgns - δnm δjs] Bj;m;s = 0?
For this to be true for every tensor B, we would need to have
gjmgns = δnm δjs
But this is NOT TRUE.
Conclusion: My claim of unique tensorization stated near (15.2.5) is wrong!
Something then is wrong with my proof. Let's then step through my proof:
{...}1a = {...}2a when both sides are evaluated in Cartesian x-space . (15.2.5)
(Bj;j);a = Bj;a;j // realization of the above!
"One can then transform both objects to x'-space in the usual manner,
{...}'1a = Rab{...}1b and {...}'2a = Rab{...}2b . " (15.2.6)
(B'j;j);a = Rab(Bj;j);b and B'j;a;j = Rab Bj;b;j
"Therefore, since {...}1a = {...}2a in x-space, one must have {...}'1a = {...}'2a in x'-space. "
Well, I would write
(B'j;j);a = Rab(Bj,j),b B'j;a;j = Rab Bj,b,j (*)
And it is true the that right sides are indeed the same since in Cartesian space
(Bj,j),b = ∂b(∂jBj) Bj,b,j = ∂j(∂bBj)
Since the right sides of (*) are exactly the same in Cartesian space, the left sides must also be equal and we get
(B'j;j);a = B'j;a;j
So I see the claimed logic of my proof!