# Coefficient of det($xI+A$)

I want to know how to prove the fact that the coefficient of $x^k$ in $\det(xI+A)$ is the sum of all $(n-k)\times (n-k)$ submatrices of $A$, an $n \times n$ matrix.

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A is n by n matrix –  user67458 Mar 20 '13 at 17:21
This is going to be very very difficult, because it is not true. Consider what you get for $k=n-1$. –  Mariano Suárez-Alvarez Mar 20 '13 at 17:24
Please note the $\LaTeX$ formatting on your question. Also, you can edit your question, no need to add conditions in the comments. –  Andreas Caranti Mar 20 '13 at 17:25
Is that a fact? The sum of all $(n-k)\times(n-k)$ submatrices of $A$ is an $(n-k)\times(n-k)$ matrix, whereas the coefficient of $x^k$ in $\det(xI+A)$ is a scalar. They aren't even the same kind of thing unless $n-k=1$. –  Henning Makholm Mar 20 '13 at 17:26
@user67458, that still does not work. You should do by hand the case $n=2$ at least! –  Mariano Suárez-Alvarez Mar 20 '13 at 17:30

This is true, except you only want to consider the determinants of the principal submatrices: i.e. the $[A_{i,j}]_{i,j \in I}$ for some set $I$ (the same one for $i$'s and $j$'s).
However you approach the issue, I think it leads to some mundane/vague transformations. You can, for instance, write out the formula for $\det(xI + A)$ as a sum over permutations, and open the brackets of the terms like $(x+A_{i,i})$. Now, to get $x^k$, you need to choose precisely $k$ occurrences of $x$: this means, you have to fix $k$ indices $i_1,i_2,\dots,i_k$, and the corresponding permutation $\pi$ has to map $i_l$ to $i_l$ (for $l=1,2,\dots,k$). On the remaining indices, you need not to get any $x$'s, which means you can essentially forget the $x$'s were there in the first place, and just consider the entries of $A$. If you sum over all possible ways in which you get $x^k$ out of the indices $i_1,i_2,\dots,i_k$, you recover the formula for the determinant of the matrix $[A_{i,j}]_{i,j \in I}$ where $I = \{1,2,\dots,n\} \setminus \{i_1,i_2,\dots,i_k\}$. Sum over all choices of $i_1,i_2,\dots,i_k$ to get your claim.