Divisibility constraints on degrees of factor maps
Paul B. Trow · Proceedings of the American Mathematical Society · 1991
We show that the degree of a finite-to-one factor map $f:{\sum _A} \to {\sum _B}$ between shifts of finite type is constrained by the factors of ${\chi _A}$ and ${\chi _B}$. A special case of these constraints is that if $^*B$, then the degree of $f$ is a unit in $\mathbb {Z}[1/{\det ^*}B]$ (where $^*A$ is the rank of the Jordan form away from 0 of $A$, and ${\det ^*}B$ is the determinant of the Jordan form away from 0 of $B$).