On the boundary of totally positive upper triangular matrices Original Research Article

Let B+ subset of GLn (R) denote the subgroup of upper triangular n × n matrices with positive entries on the main diagonal. A matrix M set membership, variant B+ is called totally positive if the determinants of all its minors not containing a row or column lying completely under the main diagonal are positive. We give a simple determinantal equation for the boundary of all positive upper triangular matrices in B+.