Vol. 6, No. 3, 2012

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ISSN: 1944-7833 (e-only)
ISSN: 1937-0652 (print)
On the weak Lefschetz property for powers of linear forms

Juan C. Migliore, Rosa M. Miró-Roig and Uwe Nagel

Vol. 6 (2012), No. 3, 487–526
Abstract

Ideals generated by prescribed powers of linear forms have attracted a great deal of attention recently. In this paper we study properties that hold when the linear forms are general, in a sense that we make precise. Analogously, one could study so-called “general forms” of the same prescribed degrees. One goal of this paper is to highlight how the differences between these two settings are related to the weak Lefschetz property (WLP) and the strong Lefschetz property (SLP). Our main focus is the case of powers of r + 1 general linear forms in r variables. For four variables, our results allow the exponents to all be different, and we determine when the WLP holds and when it does not in a broad range of cases. For five variables, we solve this problem in the case where all the exponents are equal (uniform powers), and in the case where one is allowed to be greater than the others. For evenly many variables ( 6) we solve the case of uniform powers, and in particular we prove half of a recent conjecture by Harbourne, Schenck and Seceleanu by showing that for evenly many variables, an ideal generated by d-th powers of r + 1 general linear forms fails the WLP if and only if d > 1. For uniform powers of an odd number of variables, we also give a result for seven variables, missing only the case d = 3. Our approach in this paper is via the connection (thanks to Macaulay duality) to fat point ideals, together with a reduction to a smaller projective space, and the use of Cremona transformations.

Keywords
weak Lefschetz property, artinian algebra, powers of linear forms, fat points
Mathematical Subject Classification 2010
Primary: 14C20, 13D40
Secondary: 13E10, 13C13, 13D02
Milestones
Received: 13 September 2010
Revised: 9 June 2011
Accepted: 19 July 2011
Published: 5 July 2012
Authors
Juan C. Migliore
Department of Mathematics
University of Notre Dame
255 Hurley Hall
Notre Dame, IN 46556
United States
Rosa M. Miró-Roig
Facultat de Matemàtiques
Departament d’Algebra i Geometria
University of Barcelona
Gran Via de les Corts Catalanes 585
08007 Barcelona
Spain
Uwe Nagel
Department of Mathematics
University of Kentucky
715 Patterson Ofiice Tower
Lexington, KY 40506-0027
United States