a1 University of Washington, Seattle, Washington 98195, USA
We sharpen a result of Boyle, Marcus and Trow as follows. An aperiodic shift of finite type ΣA factors onto another ΣB with equal entropy by a 1-to-l almost everywhere right-closing map if and only if (1) the dimension group for ΣB is a quotient of that for ΣA; and (2) ΣA and ΣB satisfy the trivial periodic point condition for existence of a shift-commuting map from ΣA to ΣB.
(Received May 16 1989)
† Supported by an NSF Post-doctoral Fellowship in the Mathematical Sciences.