# Formation of a positive-definite matrix via a positive-semidefinite one

Let the square symmetric matrix $L\in\mathbb{R}^{n\times n}$ be positive semi-definite with vector $1_n$ spanning its null-space (i.e., vector $1_n$ is the eigenvector of $L$ corresponding to the eigenvalue $0$). Consider $L$ to have positive diagonal entries, while the negative entries appear only in off-diagonal positions.

A matrix $X\in\mathbb{n\times k}$, $k<n$, is sought such that $$X^TLX,$$ is positive-definite. Can this be achieved, and, if so, which conditions need to be satisfied by columns of $X$ in order for $X^TLX$ to be positive definite?

I suppose that $X^T1_n=0_m$ needs be satisfied, but I'm not sure if this is sufficient (and correct).

Note that the positive definiteness of a symmetric matrix $A$ is equivalent to exclusively positive spectrum of $A$.

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Trivial case is $\mathbf{L}$ being a zero matrix. – chaohuang Sep 14 '12 at 15:20
@chaohuang Well, why should one be concerned with $L$ being a zero matrix? – user506901 Sep 14 '12 at 15:27

You can take the columns of $X$ to be an orthonormal family of vectors :
for $k=1$, you just need to take $X$ a vector such that $X^TLX>0$. If $k>1$, suppose you have found $k-1$ vectors which are orthonormal (so $X^TLY=0$ and $X^TLX=1$ for $X,Y$ two vectors in your family).
Take the subspace of $\mathbb{R}^n$ orthogonal (with respect to $L$) to the span of your family. On this space, either $L$ is $0$ or you can find $X$ such that $X^TLX>0$, and this gives you a $k$-th element for your family.
Once you've done it enough times, you put all your vectors as the columns of your $(n\times k)$-matrix that you were looking for.
Let me summarize: I could simply pick a set of $k$ vectors in $\mathbb{R}^n$, and perform Gram-Schmidt orthonormalization to obtain $k$ orthonormal vectors. What about each column $X^i$ of $X$ satisfying $1_n^TX^i=0$? – user506901 Sep 14 '12 at 17:12
When doing Gram-Schmidt with a semi-definite quadratic form, you may get vectors with $X^TLX=0$ (so you cannot normalize them). You need to be careful about that. Otherwise, this is indeed the basic idea of my construction. – zozoens Sep 14 '12 at 20:09