The trianguline variety, tangent spaces and the Grothendieck-Springer resolution

TL;DR

This paper investigates the structure of the trianguline variety related to crystalline points, introduces a combinatorial property of permutation pairs, and provides a formula for tangent space dimensions at certain points, advancing understanding of p-adic Galois representations.

Contribution

It defines a combinatorial property of permutation pairs linked to Bruhat order, proves a conjecture on Schubert cell intersections, and derives tangent space dimension formulas for the trianguline variety at crystalline points.

Findings

Most permutation pairs are 'good' and satisfy the property.

A formula for tangent space dimension at generic crystalline points is established.

Counter-examples are provided for 'bad' pairs where the conjecture fails.

Abstract

By the work of Breuil-Hellmann-Schraen, we know that the trianguline variety contains crystalline companion points which are parametrised by pairs (w,w_sat) of permutations. We first define and study a certain combinatorial property of a pair (w',w) of permutations, linked to the Bruhat order, in the context of Weyl groups of root systems. We call good pairs the pairs satisfying this property (which is the vast majority of pairs) and bad pairs the other ones. We then give an exact formula for the dimension of the tangent space to the trianguline variety at (generic) crystalline companion points such that (w,w_sat) is a good pair. The method (due to Breuil-Hellmann-Schraen) is to first compute an analogous dimension for a local model of the trianguline variety built out of Grothendieck's simultaneous resolution. To achieve this, we prove a conjecture of Breuil-Hellmann-Schraen, describing the intersection of the closure of a Schubert cell with another Schubert cell on this local model (in the context of an arbitrary split reductive group), when this pair of cells is parametrised by a good pair of permutations. We give counter-examples to this conjecture for an infinite family of pairs of cells (associated to bad pairs).