Multivariate blowup-polynomials of graphs

In recent joint work (2021), we introduced a novel multivariate polynomial attached to every metric space - in particular, to every finite simple connected graph $G$ - and showed it has several attractive properties. First, it is multi-affine and real-stable (leading to a hitherto unstudied delta-matroid for each graph $G$). Second, the polynomial specializes to (a transform of) the characteristic polynomial $chi_{D_G}$ of the distance matrix $D_G$; as well as recovers the entire graph, where $chi_{D_G}$ cannot do so. Third, the polynomial encodes the determinants of a family of graphs formed from $G$, called the blowups of $G$. In this short note, we exhibit the applicability of these tools and techniques to other graph-matrices and their characteristic polynomials. As a particular case, we will see that the adjacency characteristic polynomial $chi_{A_G}$ is in fact the shadow of a richer multivariate blowup-polynomial, which is similarly multi-affine and real-stable. Moreover, this polynomial encodes not only the aforementioned three properties, but also yields additional information for specific families of graphs.

The blowup-polynomial of a metric space: connections to stable polynomials, graphs and their distance spectra

To every finite metric space $X$, including all connected unweighted graphs with the minimum edge-distance metric, we attach an invariant that we call its blowup-polynomial $p_X({ n_x : x in X })$. This is obtained from the blowup $X[{bf n}]$ - which contains $n_x$ copies of each point $x$ - by computing the determinant of the distance matrix of $X[{bf n}]$ and removing an exponential factor. We prove that as a function of the sizes $n_x$, $p_X({bf n})$ is a polynomial, is multi-affine, and is real-stable. This naturally associates a delta-matroid to each metric space $X$ (and another delta-matroid to every tree), which also seem to be hitherto unexplored. We moreover show that the homogenization at $-1$ of $p_X({bf n})$ is Lorentzian (or strongly/completely log-concave), if and only if the normalization of $p_X(-{bf n})$ is strongly Rayleigh, if and only if a modification of the distance matrix of $X$ is positive semidefinite. We next specialize to the case of $X = G$ a connected unweighted graph - so $p_G$ is partially symmetric in ${ n_v : v in V(G) }$ - and show two further results: (a) We show that the univariate specialization $u_G(x) := p_G(x,dots,x)$ is a transform of the characteristic polynomial of the distance matrix $D_G$; this connects the blowup-polynomial of $G$ to the well-studied distance spectrum of $G$. (b) We show that the polynomial $p_G$ is indeed a graph invariant, in that $p_G$ and its symmetries recover the graph $G$ and its isometries, respectively.

Sign non-reversal property for totally non-negative and totally positive matrices, and testing total positivity of their interval hull

A matrix $A$ is totally positive (or non-negative) of order $k$, denoted $TP_k$ (or $TN_k$), if all minors of size $leq k$ are positive (or non-negative). It is well-known that such matrices are characterized by the variation diminishing property together with the sign non-reversal property. We do away with the former, and show that $A$ is $TP_k$ if and only if every submatrix formed from at most $k$ consecutive rows and columns has the sign non-reversal property. In fact this can be strengthened to only consider test vectors in $mathbb{R}^k$ with alternating signs. We also show a similar characterization for all $TN_k$ matrices - more strongly, both of these characterizations use a single vector (with alternating signs) for each square submatrix. These characterizations are novel, and similar in spirit to the fundamental results characterizing $TP$ matrices by Gantmacher-Krein [Compos. Math. 1937] and $P$-matrices by Gale-Nikaido [Math. Ann. 1965]. As an application, we study the interval hull $mathbb{I}(A,B)$ of two $m times n$ matrices $A=(a_{ij})$ and $B = (b_{ij})$. This is the collection of $C in mathbb{R}^{m times n}$ such that each $c_{ij}$ is between $a_{ij}$ and $b_{ij}$. Using the sign non-reversal property, we identify a two-element subset of $mathbb{I}(A,B)$ that detects the $TP_k$ property for all of $mathbb{I}(A,B)$ for arbitrary $k geq 1$. In particular, this provides a test for total positivity (of any order), simultaneously for an entire class of rectangular matrices. In parallel, we also provide a finite set to test the total non-negativity (of any order) of an interval hull $mathbb{I}(A,B)$.

Distance matrices of a tree: two more invariants, and in a unified framework

Graham-Pollak showed that for $D = D_T$ the distance matrix of a tree $T$, det$(D)$ depends only on its number of edges. Several other variants of $D$, including directed/multiplicative/$q