Monday, 25 February 2008

ap.analysis of pdes - Rellich-Necas identity

(Boo! I tried to post this in a comment to Ady, but the HTML Math won't parse right. So here goes. Sorry about the really long equation being broken up not very neatly.)



Googling Rellich-Necas turns up a bunch of recent papers by LUIS ESCAURIAZA in which the identities are used. But as far as I can tell the identity is just a simple differential equality obtained from symbolic manipulation of terms. The following seems to be a straight-forward version of the identity: let A=(Aij) be a symmetric bilinear form (with variable coefficients) on RN, v a vector field, u a function, and delta denoting the Euclidean divergence, we have



delta(A(nablau,nablau)v)=2delta(v(u)A(nablau))+delta(v)A(nablau,nablau)
2A(nablau)cdotnablavcdotnablau2v(u)delta(A(nablau))+v(A)(nablau,nablau)



Where v(u) is the partial derivative of u in the direction of v, and A(nablau)cdotnablavcdotnablau is, in coordinates, partialiuAijpartialjvkpartialku with implied summation, and v(A) is the symmetric bilinear form obtained by taking the v partial derivative of the coefficients of A.



Verifying that the identity is true should just be a basic application of multivariable calculus.

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