A shuffle of two strings is formed by interleaving the characters into a new string, keeping the characters of each string in order. A string is a square if it is a shuffle of two identical strings. There is a known polynomial time dynamic programming algorithm to determine if a given string z is the shuffle of two given strings x, y; however, it has been an open question whether there is a pol...

One of the corollaries of Ornstein’s isomorphism theorem is that if (Y, S, ν) is an invertible measure preserving transformation and (Y, S, ν) is isomorphic to a Bernoulli shift then (Y, S, ν) is isomorphic to a Bernoulli shift. In this paper we show that noninvertible transformations do not share this property. We do this by exhibiting a uniformly 2-1 endomorphism (X, σ, μ) which is not isomor...