Involutions and characters of upper triangular matrix groups

We study the realizability over R of representations of the group U(n) of upper-triangular n× n matrices over F2. We prove that all the representations of U(n) are realizable over R if n ≤ 12, but that if n ≥ 13, U(n) has representations not realizable over R. This theorem is a variation on a result that can be obtained by combining work of J. Arregi and A. Vera-López and of the authors, but the proof of the theorem in this paper is much more natural.