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Let $f: \mathbb{R}^{n\times n} \to \mathbb{R}$ be a function that receives a square matrix and spits out a scalar. Does there exist a function $f$ such that the gradient $\nabla_Xf(X) = \mathrm{Tr}[(I-X)^{-1}] \cdot g(X)$? Where $I$ is the identity matrix, $\mathrm{Tr}$ denotes the trace, and $g(X)$ is a function that returns a matrix.

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  • $\begingroup$ Why wouldn't $g(X)=trace[(I-X)^{-1}]^{-1} \nabla_X f(X)$ work? $\endgroup$
    – jlewk
    Commented Oct 26, 2021 at 18:13

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This is an answer to the question as originally formulated, whether there exist a function $f(X)$ with $\nabla_X f(X)=\mathrm{Tr}[(I-X)^{-1}g(X)]$.

The gradient $\nabla_X$ will give you an $n\times n$ matrix, while the trace is a scalar, so $\nabla_Xf(X)$ cannot be equal to a trace; to get a scalar derivative you can let $X$ depend on a parameter $t$, and take $$f(X(t))=-\log\,{\rm det}(I-X(t))\Rightarrow \frac{d}{dt}f(X(t))={\rm tr}\,[(I-X(t))^{-1}X'(t)].$$

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  • $\begingroup$ You are right, Carlo. I realized I asked a non-sense. I have reformulated the question, basically, I am interested in a function that has a gradient with a factor of $\mathrm{Tr}[(I-X)^{-1}]$ $\endgroup$
    – Bee
    Commented Oct 26, 2021 at 16:35

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