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The Geometry of the Weak Lefschetz Property and Level Sets of Points
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- Journal:
- Canadian Journal of Mathematics / Volume 60 / Issue 2 / 01 April 2008
- Published online by Cambridge University Press:
- 20 November 2018, pp. 391-411
- Print publication:
- 01 April 2008
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- Article
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In a recent paper, F. Zanello showed that level Artinian algebras in 3 variables can fail to have the Weak Lefschetz Property
$(\text{WLP})$, and can even fail to have unimodal Hilbert function. We show that the same is true for the Artinian reduction of reduced, level sets of points in projective 3-space. Our main goal is to begin an understanding of how the geometry of a set of points can prevent its Artinian reduction from having 
$\text{WLP}$, which in itself is a very algebraic notion. More precisely, we produce level sets of points whose Artinian reductions have socle types 3 and 4 and arbitrary socle degree ≥ 12 (in the worst case), but fail to have $\text{WLP}$. We also produce a level set of points whose Artinian reduction fails to have unimodal Hilbert function; our example is based on Zanello's example. Finally, we show that a level set of points can have Artinian reduction that has $\text{WLP}$ but fails to have the Strong Lefschetz Property. While our constructions are all based on basic double 
$G$-linkage, the implementations use very different methods.
