Some characterizations of singular components of springer fibers in the two-column case

Let u be a nilpotent endomorphism of a finite dimensional ℂ-vector space. The set F _u of u-stable complete flags is a projective algebraic variety called a Springer fiber. Its irreducible components are parameterized by a set of standard tableaux. We provide three characterizations of the singular components of F _u in the case u ² = 0. First, we give the combinatorial description of standard tableaux corresponding to singular components. Second, we prove that a component is singular if and only if its Poincaré polynomial is not palindromic. Third, we show that a component is singular when it has too many intersections of codimension one with other components. Finally, relying on the second criterion, we infer that, for u general, whenever F _u has a singular component, it admits a component whose Poincaré polynomial is not palindromic. This work relies on a previous criterion of singularity for components of F _u in the case u ² = 0 by the first author and on the description of the B-orbit decomposition of orbital varieties of nilpotent order two by the second author.