The measure transfer for subshifts induced by a morphism of free monoids
Every non-erasing monoid morphism $\sigma: {\cal A}^* \to {\cal B}^*$ induces a measure transfer map $\sigma_X^{\cal M}: {\cal M}(X) \to {\cal M}(\sigma(X))$ between the measure cones ${\cal M}(X)$ and ${\cal M}(\sigma(X))$, associated to any subshift $X \subseteq {\cal A}^\mathbb Z$ and its image subshift $\sigma(X) \subseteq {\cal B}^\mathbb Z$ respectively. We define and study this map in detail and show that it is continuous, linear and functorial. It also turns out to be surjective. Furthermore, an efficient technique to compute the value of the transferred measure $\sigma_X^{\cal M}(\mu)$ on any cylinder $[w]$ (for $w \in {\cal B}^\mathbb Z$) is presented. Theorem: If a non-erasing morphism $\sigma: {\cal A}^* \to {\cal B}^*$ is injective on the shift-orbits of some subshift $X \subseteq {\cal A}^\mathbb Z$, then $\sigma^{\cal M}_X$ is bijective. The assumption on $\sigma$ that it is "injective on the shift-orbits of $X$" is strictly weaker than "recognizable in $X$", and strictly stronger than "recognizable for aperiodic points in $X$". The last assumption does in general not suffice to obtain the injectivity of the measure transfer map $\sigma_X^{\cal M}$.