Linear Algebra : Gradients of the Determinant

Study concepts, example questions & explanations for Linear Algebra

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Example Questions

Example Question #71 : Linear Algebra

Which of the following expressions is one for the gradient of the determinant of an \(\displaystyle n \times n\) matrix \(\displaystyle A\)?

Possible Answers:

\(\displaystyle (adj(A))^{-1}\)

\(\displaystyle \mathbf{x}^{-1}A\mathbf{x}, (\|\mathbf{x}\|=1)\)

\(\displaystyle (adj(A))^T\)

None of the other answers

Correct answer:

\(\displaystyle (adj(A))^T\)

Explanation:

The expression for the determinant of \(\displaystyle A\) using co-factor expansion (along any row) is

\(\displaystyle |A| = \sum_{i=1}^n(-1)^{i+j}A_{ij}M_{ij}\)

In order to find the gradient of the determinant, we take the partial derivative of the determinant expression with respect to some entry \(\displaystyle A_{kl}\) in our matrix, yielding \(\displaystyle (-1)^{k+l}M_{kl} = (adj(A))_{kl} = (adj(A))^T\).

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