Published online by Cambridge University Press: 08 March 2016
Fix   $d\geq 2$ . Given a finite undirected graph
 $d\geq 2$ . Given a finite undirected graph   ${\mathcal{H}}$  without self-loops and multiple edges, consider the corresponding ‘vertex’ shift,
 ${\mathcal{H}}$  without self-loops and multiple edges, consider the corresponding ‘vertex’ shift,   $\text{Hom}(\mathbb{Z}^{d},{\mathcal{H}})$ , denoted by
 $\text{Hom}(\mathbb{Z}^{d},{\mathcal{H}})$ , denoted by   $X_{{\mathcal{H}}}$ . In this paper, we focus on
 $X_{{\mathcal{H}}}$ . In this paper, we focus on   ${\mathcal{H}}$  which is ‘four-cycle free’. There are two main results of this paper. Firstly, that
 ${\mathcal{H}}$  which is ‘four-cycle free’. There are two main results of this paper. Firstly, that   $X_{{\mathcal{H}}}$  has the pivot property, meaning that, for all distinct configurations
 $X_{{\mathcal{H}}}$  has the pivot property, meaning that, for all distinct configurations   $x,y\in X_{{\mathcal{H}}}$ , which differ only at a finite number of sites, there is a sequence of configurations
 $x,y\in X_{{\mathcal{H}}}$ , which differ only at a finite number of sites, there is a sequence of configurations   $x=x^{1},x^{2},\ldots ,x^{n}=y\in X_{{\mathcal{H}}}$  for which the successive configurations
 $x=x^{1},x^{2},\ldots ,x^{n}=y\in X_{{\mathcal{H}}}$  for which the successive configurations   $x^{i},x^{i+1}$  differ exactly at a single site. Secondly, if
 $x^{i},x^{i+1}$  differ exactly at a single site. Secondly, if   ${\mathcal{H}}$  is connected ,then
 ${\mathcal{H}}$  is connected ,then   $X_{{\mathcal{H}}}$  is entropy minimal, meaning that every shift space strictly contained in
 $X_{{\mathcal{H}}}$  is entropy minimal, meaning that every shift space strictly contained in   $X_{{\mathcal{H}}}$  has strictly smaller entropy. The proofs of these seemingly disparate statements are related by the use of the ‘lifts’ of the configurations in
 $X_{{\mathcal{H}}}$  has strictly smaller entropy. The proofs of these seemingly disparate statements are related by the use of the ‘lifts’ of the configurations in   $X_{{\mathcal{H}}}$  to the universal cover of
 $X_{{\mathcal{H}}}$  to the universal cover of   ${\mathcal{H}}$  and the introduction of ‘height functions’ in this context.
 ${\mathcal{H}}$  and the introduction of ‘height functions’ in this context.