the positions at which they fail to be periodic. We prove that if a $\mathbb Z^ 2$-subshift
contains points whose smallest period is arbitrarily large, then it contains an aperiodic point.
This lets us characterise the computational difficulty of deciding if an $\mathbb Z^ 2$-subshift
of finite type contains an aperiodic point. Another consequence is that $\mathbb Z^ 2$-
subshifts with no aperiodic point have a very strong dynamical structure and are almost …