it is known that variation is defined by following:
but could anyone tell me why the integral symbol disappears after following functional derivative?
Answer
Define functional G[g] := ∫d4x √−g(x). Method 1: ∫d4x δG[g]δgμν(x)δgμν(x) = δG[g] (0)= ∫d4x ∂√−g(x)∂gμν(x)δgμν(x). Method 2: δG[g]δgμν(y) (0)= ∫d4x δ√−g(x)δgμν(y) = ∫d4x ∂√−g(x)∂gκλ(x)δgκλ(x)δgμν(y) = ∫d4x ∂√−g(x)∂gμν(x)δ4(x−y) = ∂√−g(y)∂gμν(y).
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