Can we write unitary matrices as positive linear combinations of Hermitian matrices? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-20T16:34:02Z http://mathoverflow.net/feeds/question/49624 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/49624/can-we-write-unitary-matrices-as-positive-linear-combinations-of-hermitian-matric Can we write unitary matrices as positive linear combinations of Hermitian matrices? Colin Tan 2010-12-16T09:47:46Z 2010-12-16T11:28:15Z <p>The space $M_n:=M_n(\mathbb{C})$ of complex $n\times n$ matrices has the structure of a finite-dimensional complex vector space.</p> <p>The space of Hermitian matrices forms a cone in this vector space $M_n$. (By a Hermitian matrix, I mean just a Hermitian symmetric matrix. I make no restrictions on being positive-definiteness.) Suppose you are given finitely many Hermitian matrices $A_1,\ldots, A_k$. The conical hull $con(A_1,\ldots, A_k)$ consists of all positive linear combinations of $A_1,\ldots, A_k$.</p> <p>Can we find some conditions on $A_1,\ldots, A_k$ such that the conical hull $con(A_1,\ldots, A_k)$ contains at least one unitary matrix? I hope this is not too vague for MathOverflow.</p> <p><b>Edit:</b> Let me add more background, as requested by Suvrit. Suppose I have linear forms $p_1,\ldots, p_k$, i.e. homogeneous degree 1 polynomials in the variables $x_1,\ldots, x_n$. I am given real coefficients $c_1,\ldots, c_k$, which are either $1$ or $-1$. Taking linear combinations gives me a quadratic form $c_1 |p_1|^2+\cdots + c_k|p_k|^2$. I want to know when this quadratic form is a positive-definite form in the variables $x_1,\ldots, x_n$.</p> <p>The norms $|p_t|^2=\sum a_{ij}x_i\bar{x_j}$ are polynomials whose coefficients $(a_{ij})$ form a Hermitian matrix. Using this correspondence, I rephrased the question in terms of matrices, hoping to gain geometric intuition from the convex geometry. </p>