Syzygy bundles on P 2 and the weak Lefschetz property

Let K be an algebraically closed field of characteristic zero and let I= (f₁ fₙ) be a homogeneous R_+-primary ideal in R: =KX, Y, Z. If the corresponding syzygy bundle (f₁ fₙ) on the projective plane is semistable, we show that the Artinian algebra R/I has the Weak Lefschetz property if and only if the syzygy bundle has a special generic splitting type. As a corollary we get the result of Harima et alt. , that every Artinian complete intersection (n=3) has the Weak Lefschetz property. Furthermore, we show that an almost complete intersection (n=4) does not necessarily have the Weak Lefschetz property, answering negatively a question of Migliore and Miró-Roig. We prove that an almost complete intersection has the Weak Lefschetz property if the corresponding syzygy bundle is not semistable.