We study the thermodynamic formalism for particular types of sub-additive sequences on a class of subshifts over countable alphabets. The subshifts we consider include factors of irreducible countable Markov shifts under certain conditions. We show the variational principle for topological pressure. We also study conditions for the existence and uniqueness of invariant ergodic Gibbs measures and the uniqueness of equilibrium states. As an application, we extend the theory of factors of (generalized) Gibbs measures on subshifts on finite alphabets to that on certain subshifts over countable alphabets.∞ n=0 B n (X), i.e., the union of all allowable words of X and the empty word ε.We now define several notions of specification that generalize the one first introduced by Bowen [Bo] with the purpose of proving that there exits a unique measure of maximal entropy for a large class of compact subshifts. Our definitions are given in terms of the language of X.Definition 2.1. We say that a subshift (X, σ) on a countable alphabet is irreducible if for any allowable words u, v ∈ B(X), there exists an allowable word w ∈ B(X) such that uwv ∈ B(X).Definition 2.2. We say that a subshift (X, σ) on a countable alphabet has the weak specification property if there exists p ∈ N such that for any allowable words u, v ∈ B(X), there exist 0 ≤ k ≤ p and w ∈ B k (X) such that uwv ∈ B(X). If in addition, k = p for any u and v, then X has the strong specification property. We call such p a weak (strong, respectively) specification number. Definition 2.3. A subshift (X, σ) is finitely irreducible if there exist p ∈ N and a finite subset W 1 ⊂ p n=0 B n (X) such that for every u, v ∈ B(X), there exists w ∈ W 1 such that uwv ∈ B(X).
GODOFREDO IOMMI, CAMILO LACALLE, AND YUKI YAYAMADefinition 2.4. A subshift (X, σ) is finitely primitive if there exist p ∈ N and a finite subset W 1 ⊂ B p (X) such that for every u, v ∈ B(X), there exists w ∈ W 1 such that uwv ∈ B(X).Remark 2.1. Note that the weak specification property does not imply topologically mixing. However, if (Σ, σ) is a topologically mixing subshift of finite type defined on a finite alphabet with the weak specification property, then it has the strong specification property (see [J1, Lemma 3.2]). The class of general shifts on finite alphabets with the weak specification property include irreducible sofic shifts (see [J1] and Definition 2.11).As it is clear from the definition, the notion of finitely primitive (see Definition 2.4) is essentially the same as that of specification introduced by Bowen [Bo] in a non-compact symbolic setting. There is a closely related class of countable Markov shifts studied by Sarig [S3].Definition 2.5. A countable Markov shift (Σ, σ) is said to satisfy the big images and preimages property (BIP property) if there exists {b 1 , b 2 , . . . , b n } in the alphabet S such that for every a ∈ S there exist i, j ∈ {1, . . . , n} such that t bia t abj = 1.Remark 2.2. If the countable Markov shift (Σ, σ) satisfies the BIP property, then for every symbol in the a...