Not all GKK $τ$-matrices are stable
Hermitian positive definite, totally positive, and nonsingular M-matrices enjoy many common properties, in particular: (A) positivity of all principal minors, (B) weak sign symmetry, (C) eigenvalue monotonicity, (D) positive stability. The class of GKK matrices is defined by properties (A) and (B), whereas the class of nonsingular $τ$-matrices by (A) and (C). It was conjectured that: (A), (B) implies (D) [D. Carlson, J. Res. Nat. Bur. Standards Sect. B 78 (1974) 1-2], (A), (C) implies (D) [G.M. Engel and H. Schneider, Linear and Multilinear Algebra 4 (1976) 155-176], (A), (B) implies a property stronger than (D) [R. Varga, Numerical Methods in Linear Algebra, 1978, pp. 5-15], (A), (B), (C) implies (D) [D. Hershkowitz, Linear Algebra Appl. 171 (1992) 161-186]. We describe a class of unstable GKK $τ$-matrices, thus disproving all four conjectures.