Published online by Cambridge University Press: 29 January 2016
We consider a non-uniquely ergodic dynamical system given by a $\mathbb{Z}^{l}$-action (or
$(\mathbb{N}\cup \{0\})^{l}$-action)
$\unicode[STIX]{x1D70F}$ on a non-empty compact metrisable space
$\unicode[STIX]{x1D6FA}$, for some
$l\in \mathbb{N}$. Let (D) denote the following property: the graph of the restriction of the entropy map
$h^{\unicode[STIX]{x1D70F}}$ to the set of ergodic states is dense in the graph of
$h^{\unicode[STIX]{x1D70F}}$. We assume that
$h^{\unicode[STIX]{x1D70F}}$ is finite and upper semi-continuous. We give several criteria in order that (D) holds, each of which is stated in terms of a basic notion: Gateaux differentiability of the pressure map
$P^{\unicode[STIX]{x1D70F}}$ on some sets dense in the space
$C(\unicode[STIX]{x1D6FA})$ of real-valued continuous functions on
$\unicode[STIX]{x1D6FA}$, level-two large deviation principle, level-one large deviation principle, convexity properties of some maps on
$\mathbb{R}^{n}$ for all
$n\in \mathbb{N}$. The one involving the Gateaux differentiability of
$P^{\unicode[STIX]{x1D70F}}$ is of particular relevance in the context of large deviations since it establishes a clear comparison with another well-known sufficient condition: we show that for each non-empty
$\unicode[STIX]{x1D70E}$-compact subset
$\unicode[STIX]{x1D6F4}$ of
$C(\unicode[STIX]{x1D6FA})$, (D) is equivalent to the existence of an infinite dimensional vector space
$V$ dense in
$C(\unicode[STIX]{x1D6FA})$ such that
$f+g$ has a unique equilibrium state for all
$(f,g)\in \unicode[STIX]{x1D6F4}\times V\setminus \{0\}$; any Schauder basis
$(f_{n})$ of
$C(\unicode[STIX]{x1D6FA})$ whose linear span contains
$\unicode[STIX]{x1D6F4}$ admits an arbitrary small perturbation
$(h_{n})$ so that one can take
$V=\text{span}(\{f_{n}+h_{n}:n\in \mathbb{N}\})$. Taking
$\unicode[STIX]{x1D6F4}=\{0\}$, the existence of an infinite dimensional vector space dense in
$C(\unicode[STIX]{x1D6FA})$ constituted by functions admitting a unique equilibrium state is equivalent to (D) together with the uniqueness of the measure of maximum entropy.