Bug in the Chow ring of a matroid

[sagetrac-jwiltshiregordon]

The Chow ring of a matroid was implemented in ticket #19587. However, there is a small error in the set of quadratic relations. The line

Q = [gens[i] * gens[i+j+1] for i,F in enumerate(flats)
     for j,G in enumerate(flats[i+1:]) if F < G or G < F]

should read

Q = [gens[i] * gens[i+j+1] for i,F in enumerate(flats)
     for j,G in enumerate(flats[i+1:]) if not (F < G or G < F)]

since the quadratic relations are imposed when two flats are incomparable.

CC: @tscrim

Component: matroid theory

Author: John Wiltshire-Gordon

Branch/Commit: 2773de6

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/21761

[tscrim]

Commit: 26d4124

[tscrim]

Author: John Wiltshire-Gordon

[tscrim]

Branch: public/matroids/fix_chow_ring_ideal-21761

[tscrim]

Reviewer: Travis Scrimshaw

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

2773de6

Fixing the quadratic ideal of the Chow ring of a matroid.

[sagetrac-git]

Changed commit from 26d4124 to 2773de6

[vbraun]

Changed branch from public/matroids/fix_chow_ring_ideal-21761 to 2773de6