Let $mathbb{F}$ be an infinite field with characteristic different from two. For a graph $G=(V,E)$ with $V={1,...,n}$, let $S(G;mathbb{F})$ be the set of all symmetric $ntimes n$ matrices $A=[a_{i,j}]$ over $mathbb{F}$ with $a_{i,j} ot=0$, $i ot=j$ if and only if $ijin E$. We show that if $G$ is the complement of a partial $k$-tree and $mgeq k+2$, then for all nonsingular symmetric $mtimes m$ matrices $K$ over $mathbb{F}$, there exists an $mtimes n$ matrix $U$ such that $U^T K Uin S(G;mathbb{F})$. As a corollary we obtain that, if $k+2leq mleq n$ and $G$ is the complement of a partial $k$-tree, then for any two nonnegative integers $p$ and $q$ with $p+q=m$, there exists a matrix in $S(G;reals)$ with $p$ positive and $q$ negative eigenvalues.
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