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feldy90
 3 years ago
Notation help: what does a comma in a subscript mean? I am given A_ij and asked to calculate A_ij,i ... Something about differentiation? I can't remember. Any help would be great, thanks!
feldy90
 3 years ago
Notation help: what does a comma in a subscript mean? I am given A_ij and asked to calculate A_ij,i ... Something about differentiation? I can't remember. Any help would be great, thanks!

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feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0\[A_{ij} = x_ix_j^3+3x_1x_2\delta_{ij}\] calculate \[A_{ij,i}\] is what I mean

feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0Not looking for an answer, just what the question is asking. Thanks.

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1It's notation used for partial differentiation. Inside first.

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1\[f_{x,y}=\delta\frac{\delta f}{\delta x}/\delta y\] You know, chain rule and all that fun stuff....

feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0Woah okay... Maybe I do need help then! So the above question would work out to just be\[A_{ij,i}=x_i\]? Because every j term is counted as a constant? And the Kroneker delta would just be zero, because i/=j?

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1Well hang on. I don't get why your book or whatever wouldn't use A_i,j,i and not A_ij, i... This is for differential equations, right? That's where I'm getting the subscript convention...

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1Unless it means dA/d(ij) which is possible but unlikely.

feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0Yeah I'm assuming so? The question as I have it is: "For \[A_{ij}=x_ix_j^3+3x_1x_2\delta_{ij}\] (i,j=1,2) Calculate \[A_{ij,i} \]and\[A_{kl,kl}\]"

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1Hmmm.. what course are you taking for this? It could be a matrixrelated problem...

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1and just to clarify the delta in there is that delta as in partial d or delta as in deltaepsilon delta?

feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0It's just called "Applied Mathematical Modelling" ... We did cover a few things on matrices in theis chapter, but not too in depth... I think the delta is the substitution tensor? \[\delta_{ij}=\left\{ \left(\begin{matrix}{1 'if' i=j} \\ {0 'if' i \neq j}\end{matrix}\right) \right\}\] Hmmm that didn't quite come out the way I wanted it, but basically the delta = 1 if i=j, and delta = 0 if i does not =j

vf321
 3 years ago
Best ResponseYou've already chosen the best response.1Hmm... well when you say tensor it does make me think matrix... I'd love to help, but the general uses of subscripts are to identify partial derivatives or location in a matrix (x_1,1) means top left corner. It seems to me that the use of subscripts here is a convention defined in your book. Sorry.

feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0Thanks so much for your help  much appreciated anyway :)

UnkleRhaukus
 3 years ago
Best ResponseYou've already chosen the best response.0could this \[A_{ij,i}\] mean this \[A_{ij} , A_{ii}\]

feldy90
 3 years ago
Best ResponseYou've already chosen the best response.0Ummm... Yeah, potentially? Where \[A_{ii}=A_{11}+A_{22}\]
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