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v2010.10.26 - Convex Optimization

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D.1. DIRECTIONAL DERIVATIVE, TAYLOR SERIES 671The gradient of real function g(X) : R K×L →R on matrix domain is⎡∇g(X) ⎢⎣=∂g(X)∂X 11∂g(X)∂X 21.∂g(X)∂X K1∂g(X)∂X 12· · ·∂g(X)[∇X(:,1) g(X)∂X 22.· · ·∂g(X)∂X K2· · ·∂g(X)∂X 1L∂g(X)∂X 2L.∂g(X)∂X KL⎤∈ R K×L⎥⎦∇ X(:,2) g(X)...∇ X(:,L) g(X) ] ∈ R K×1×L (1765)where gradient ∇ X(:,i) is with respect to the i th column of X . The strangeappearance of (1765) in R K×1×L is meant to suggest a third dimensionperpendicular to the page (not a diagonal matrix). The second-order gradienthas representation⎡∇ 2 g(X) ⎢⎣=∇ ∂g(X)∂X 11∇ ∂g(X)∂X 21.∇ ∂g(X)∂X K1[∇∇X(:,1) g(X)∇ ∂g(X)∂X 12· · · ∇ ∂g(X)∂X 1L∇ ∂g(X)∂X 22· · · ∇ ∂g(X)∂X 2L. .∇ ∂g(X)∂X K2· · · ∇ ∂g(X)∂X KL⎤∈ R K×L×K×L⎥⎦∇∇ X(:,2) g(X)...∇∇ X(:,L) g(X) ] ∈ R K×1×L×K×L (1766)where the gradient ∇ is with respect to matrix X .

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