The Waldschmidt constant for squarefree monomial ideals

• Published in: Journal of algebraic combinatorics Vol. 44; no. 4; pp. 875 - 904
• Main Authors: Bocci, Cristiano, Cooper, Susan, Guardo, Elena, Harbourne, Brian, Janssen, Mike, Nagel, Uwe, Seceleanu, Alexandra, Tuyl, Adam Van, Vu, Thanh
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.12.2016; Springer Nature B.V
• Subjects: Combinatorics; Computer Science; Convex and Discrete Geometry; Decomposition; Graph theory; Group Theory and Generalizations; Lattices; Linear programming; Mathematics; Mathematics and Statistics; Order; Ordered Algebraic Structures; Subspaces; Unions; Fractional chromatic number; Secondary 13A02; Primary 13F20; Graphs; Linear programming; Monomial ideals; Waldschmidt constant; Symbolic powers; 14N05; Hypergraphs; Resurgence
• ISSN: 0925-9899; 1572-9192; 1572-9192
• DOI: 10.1007/s10801-016-0693-7

Given a squarefree monomial ideal I ⊆ R = k [ x 1 , … , x n ] , we show that α ^ ( I ) , the Waldschmidt constant of I , can be expressed as the optimal solution to a linear program constructed from the primary decomposition of I . By applying results from fractional graph theory, we can then express α ^ ( I ) in terms of the fractional chromatic number of a hypergraph also constructed from the primary decomposition of I . Moreover, expressing α ^ ( I ) as the solution to a linear program enables us to prove a Chudnovsky-like lower bound on α ^ ( I ) , thus verifying a conjecture of Cooper–Embree–Hà–Hoefel for monomial ideals in the squarefree case. As an application, we compute the Waldschmidt constant and the resurgence for some families of squarefree monomial ideals. For example, we determine both constants for unions of general linear subspaces of P n with few components compared to n , and we compute the Waldschmidt constant for the Stanley–Reisner ideal of a uniform matroid.