# Derivatives of a the Matrix diagonal function

If A is a not diagonal but symetric matrix and diag() is a function such that returns the diagonal, i.e. diag(A) is a matrix of zeros except on the diagonal. Im interested in the derivative of vec(diag(A)) with respect to x where A is a function of the vector x. So, how do I do This in the best way?

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You should clarify what "A is a function of the vector x" means. Some funcion in particular? Each component of the matrix is an indepent function of x? –  leonbloy Dec 13 '12 at 19:40

The derivative of the $(i,i)$ entry of $\text{diag}(A)$ is the same as the derivative of the $(i,i)$ entry of $A$. The derivative of the $(i,j)$ entry of $\text{diag}(A)$ for $i \ne j$ is $0$.

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@ Robert. Interesting. So you are saying $\nabla diag(A)= diag(\nabla(A))$? –  user25004 Jan 8 '13 at 2:58
Of course. The derivatives are element-wise operations. –  Robert Israel Jan 9 '13 at 5:26