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We prove that, for any positive integer $m$, a segment may be partitioned into $m$ possibly degenerate or empty segments with equal values of a continuous function $f$ of a segment, assuming that $f$ may take positive and negative values, but its value on degenerate or empty segments is zero.
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We prove that any convex body in the plane can be partitioned into $m$ convex parts of equal areas and perimeters for any integer $mge 2$; this result was previously known for prime powers $m=p^k$. We also discuss possible higher-dimensional generalizations and difficulties of extending our technique to equalizing more than one non-additive function.
We show that for any compact convex set $K$ in $mathbb{R}^d$ and any finite family $mathcal{F}$ of convex sets in $mathbb{R}^d$, if the intersection of every sufficiently small subfamily of $mathcal{F}$ contains an isometric copy of $K$ of volume $1$, then the intersection of the whole family contains an isometric copy of $K$ scaled by a factor of $(1-varepsilon)$, where $varepsilon$ is positive and fixed in advance. Unless $K$ is very similar to a disk, the shrinking factor is unavoidable. We prove similar results for affine copies of $K$. We show how our results imply the existence of randomized algorithms that approximate the largest copy of $K$ that fits inside a given polytope $P$ whose expected runtime is linear on the number of facets of $P$.
Given a torus $E = S^{1} times S^{1}$, let $E^{times}$ be the open subset of $E$ obtained by removing a point. In this paper, we show that the $i$-th singular Betti number $h^{i}(mathrm{Conf}^{n}(E^{times}))$ of the unordered configuration space of $n$ points on $E^{times}$ can be computed as a coefficient of an explicit rational function in two variables. Our proof uses Delignes mixed Hodge structure on the singular cohomology $H^{i}(mathrm{Conf}^{n}(E^{times}))$ with complex coefficients, by considering $E$ as an elliptic curve over complex numbers. Namely, we show that the mixed Hodge structure of $H^{i}(mathrm{Conf}^{n}(E^{times}))$ is pure of weight $w(i)$, an explicit integer we provide in this paper. This purity statement will imply our main result about the singular Betti numbers. We also compute all the mixed Hodge numbers $h^{p,q}(H^{i}(mathrm{Conf}^{n}(E^{times})))$ as coefficients of an explicit rational function in four variables.
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.
We use category-theoretic techniques to provide two proofs showing that for a higher-rank graph $Lambda$, its cubical (co-)homology and categorical (co-)homology groups are isomorphic in all degrees, thus answering a question of Kumjian, Pask and Sims in the positive. Our first proof uses the topological realization of a higher-rank graph, which was introduced by Kaliszewski, Kumjian, Quigg, and Sims. In our more combinatorial second proof, we construct, explicitly and in both directions, maps on the level of (co-)chain complexes that implement said isomorphism. Along the way, we extend the definition of cubical (co-)homology to allow arbitrary coefficient modules.
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