Positive eigenvalues and two-letter generalized words

Christopher Hillar, Charles R. Johnson, Ilya M. Spitkovsky

A generalized word in two letters $A$ and $B$ is an expression of the form $W=A^{{\alpha }_1}{B}^{{\beta }_1}{A}^{{\alpha }_2}{B}^{{\beta }_2}...A^{{\alpha }_N}{B}^{{\beta }_N}$ in which the exponents ${\alpha }_i$, ${\beta }_i$ are nonzero real n…