Abstract. Only finitely many shift equivalence classes of non-negative aperiodic integral matrices may share a given diagonal Jordan form away from zero. The diagonal assumption is necessary.
IntroductionSubshifts of finite type are fundamental in applications of symbolic dynamics to diffeomorphisms [3], ergodic theory [4], and coding theory [2]. These subshifts are defined by square non-negative integral matrices [16], and may be studied by way of invariants of matrices which define topologically conjugate subshifts. We will refer to such invariants as 'invariants of conjugacy'.In a fundamental paper [16], Williams introduced two equivalence relations on square non-negative integral matrices: shift equivalence and strong shift equivalence. Strong shift equivalence is a complete but highly non-computable invariant of conjugacy. Shift equivalence is an invariant of conjugacy which is conjectured to be complete and which is more computable-it is often practical to decide if two matrices are shift equivalent, and significant partial results suggest there is a general decision procedure [7]. In addition, from Krieger's work we find shift equivalence intimately related to dimension groups [9] and the construction of factor maps [10].The Jordan form away from zero is a strictly weaker invariant of conjugacy. It is still a very strong one; it determines the entropy and zeta function, classifies irreducible subshifts of finite type up to 'stable weak isomorphism ' [10] and severely constrains equal-entropy factors [8]. Moreover, it is completely computable. To understand the structure of the class of subshifts of finite type, we should in particular understand how shift equivalence refines this Jordan equivalence relation.In this paper, we find that only finitely many shift equivalence classes of aperiodic matrices may share a given diagonal Jordan form away from zero. On the other hand, examples are provided in which infinitely many aperiodic non-negative integral matrices, pairwise not shift equivalent, share the same (non-diagonal) Jordan form away from zero.