# Positive definite - Inverse of sparse symmetric matrix

Consider a matrix $P\in \mathbb{R}^{n\times n}$ such that at most $m<n$ elements of each column are non zero and $P$ is symmetric. I would like to find the sufficient condition(s) such that $P^{-1}$ is positive definite. Given the positions of non zero elements in $P$ what are their values such that $P$ is positive definite.

• do you need $P^{-1}$ to be sparse? Otherwise it is trivial... – Dima Pasechnik Mar 31 '16 at 20:06
• I need specific elements of $P$ to be non zero, Jacobi Rotations on a positive definite diagonal matrix might work as user251257 said. – Cauchy Apr 1 '16 at 1:54

## 1 Answer

If $P$ is an invertible real symmetric matrix, $P^{-1}$ is positive definite iff $P$ is positive definite. There are many equivalent conditions to positive definiteness.

• That is true but not sufficient for the sparsity . So How do I generate a positive definite sparse symmetric matrix? – Cauchy Mar 31 '16 at 6:43
• @Cauchy: Google is your friend. Apply random Jacobi Rotations on a positive definite diagonal matrix... – user251257 Mar 31 '16 at 14:55