# Completeness

Theorem 1. (Completeness Theorem, 1st form) Let $$\Gamma$$ be a set of formulas, $$\varphi$$ be a formula. Then $\Gamma \models \varphi \implies \Gamma \vdash \varphi$

Consistency. A set $$\Gamma$$ of $$\mathcal{L}$$-formulas is said to be consistent iff for no formula $$\phi$$ it is the case that $$\Gamma \vdash \phi$$ and $$\Gamma \vdash \lnot \phi$$.

Satisfiability. A set $$\Gamma$$ of $$\mathcal{L}$$-formulas is said to be satisfiable iff there is $$\mathcal{L}$$-structure $$\mathfrak{A}$$ and an assignment of variables $$s : V \to |\mathfrak{A}|$$ such that $\models_\mathfrak{A} \Gamma[s]$

Theorem 2. (Completeness Theorem, 2nd form) Any consistent set of formulas is satisfiable.

Proposition 3. Theorem 2 holds iff Theorem 1 holds.

Proof. $$(\Rightarrow)$$ Assume that Theorem 2 holds and $$\Gamma \models \varphi$$. By definition we have that $$\Gamma; \lnot \varphi$$ is not satisfiable. By Theorem 2, $$\Gamma; \lnot \varphi$$ must be inconsistent. By Reductio ad Absurdum, $$\Gamma \vdash \varphi$$.

$$(\Leftarrow)$$ Assume that Theorem 1 holds and $$\Gamma$$ is not satisfiable. By definition we have that $$\Gamma \models \varphi$$ for all $$\varphi$$ of $$\mathcal{L}$$ vacuously. By Theorem 1, $$\Gamma \vdash \varphi$$ for all $$\varphi$$ of $$\mathcal{L}$$, thus $$\Gamma$$ must be inconsistent.

Countability. A set $$S$$ is said to be countable iff there is a bijective map $$f : \mathbb{N} \to S$$.

We now prove a weaker form of Theorem 2, i.e., the Completeness Theorem for countable languages. Later we will see that it generalizes to uncountable languages easily.

Theorem 4. (Completeness Theorem for countable languages) Any consistent set of formulas in a countable language is satisfiable.

Proof sketch. (Henkin construction) Let $$\mathcal{L}$$ be a countable language and $$\Gamma$$ be a consistent set of formulas.

1. (Language extension) Let $$\mathcal{L}_{\bar C}$$ be the language $$\mathcal{L}$$ extended with countably many new constant symbols $$\bar C = \{ \bar c_i : i \in \mathbb{N} \}$$.
2. (Maximal consistent, Henkinized theory) Given the set $$\Gamma$$ of $$\mathcal{L}$$-formulas, we show that there is a set $$\bar\Gamma$$ of $$\mathcal{L}_{\bar C}$$-formulas such that
1. $$\bar\Gamma$$ is consistent, i.e., for no $$\theta$$ of $$\mathcal{L}_{\bar C}$$, both $$\bar\Gamma \vdash \theta$$ and $$\bar\Gamma \vdash \lnot\theta$$.
2. $$\bar\Gamma$$ is complete, i.e., for every $$\theta$$ of $$\mathcal{L}_{\bar C}$$, either $$\theta \in \bar\Gamma$$ or $$\lnot\theta \in \bar\Gamma$$.
3. $$\bar\Gamma$$ is deductively closed, i.e., if $$\bar\Gamma \vdash \theta$$ then $$\theta \in \bar\Gamma$$.
4. $$\bar\Gamma$$ has the Henkin property, i.e., if $$\exists x \varphi \in \Gamma$$, then there is a constant $$\bar c \in \bar C$$ such that $\varphi^x_{\bar c} \in \bar\Gamma$
3. (First structure) Build an intermediate structure $$\mathfrak{A}_0$$ from $$\bar\Gamma$$, where $$|\mathfrak{A}_0|$$ is the set of terms of $$\mathcal{L}_{\bar C}$$. The assignment $$s$$ maps every variable to itself. Note that $$\mathfrak{A}_0$$ is not yet a model of $$\bar\Gamma$$, since for distinct terms $$t$$ and $$t'$$ we have $$\models_{\mathfrak{A}_0} t \neq t'$$ but it may be the case that $$t = t' \in \bar\Gamma$$.
4. (Satisfiability) Define an equivalence relation $$E$$ on $$|\mathfrak{A}_0|$$ $t E t' \iff t = t' \in \bar\Gamma$ We show by induction, that for any $$\mathcal{L}_{\bar C}$$-formula $$\varphi$$, $\models_{\mathfrak{A}_0} \varphi^* [s] \iff \varphi \in \bar\Gamma$ where $$\varphi^*$$ is $$\varphi$$ with $$=$$ replaced by $$E$$ everywhere.
5. (Final structure) We take the quotient $$\mathfrak{A} = \mathfrak{A}_0 / E$$, and show by the Homomorphism Theorem, that for any $$\mathcal{L}_{\bar C}$$-formula $$\varphi$$, $\models_\mathfrak{A} \varphi[s] \iff \varphi \in \bar\Gamma$
6. (Model restriction) Restrict the model $$\mathfrak{A}$$ of language $$\mathcal{L}_{\bar C}$$ to the original language $$\mathcal{L}$$, i.e., ignore the interpretation of new constants $$\bar C$$. As $$\Gamma \subseteq \bar\Gamma$$, we know that $$\Gamma$$ is satisfiable with the $$\mathcal{L}_{\bar C}$$-structure $$\mathfrak{A}$$ and the assignment $$s$$, thus also satisfiable with the structure $$\mathfrak{A}$$ and the assignment $$s$$ restricted to the language $$\mathcal{L}$$.

Lemma 5. Given that the set $$\Gamma$$ formulas is consistent in the original language $$\mathcal{L}$$, it is also consistent as a set of formulas in the new language $$\mathcal{L}_{\bar C}$$.

Proof. (Use Generalization on Constants.)

As $$\mathcal{L}_{\bar C}$$ is countable, we enumerate all $$\mathcal{L}_{\bar C}$$ formulas as $$\{ \theta_i : i \in \mathbb{N} \}$$. Let $$T$$ be the set of finite sequences $$s$$ of pairs of $$\mathcal{L}_{\bar C}$$-formulas $s = \langle (\alpha_0, \beta_0), (\alpha_1, \beta_1), \dots, (\alpha_n, \beta_n) \rangle$ such that

1. $$\{ \alpha_i : 0 \leq i \leq n\} \cup \{ \beta_i : 0 \leq i \leq n\} \cup \Gamma$$ is consistent.
2. For each $$0 \leq i \leq n$$, $$\alpha_i$$ is either $$\theta_i$$ or $$\lnot\theta_i$$.
3. If $$\alpha_i$$ is $$\exists x \varphi$$, then $$\beta_i$$ is the formula $\varphi^x_{\bar c_k}$ where $$k$$ is minimal such that $$\bar c_k$$ does not appear in $$\varphi$$ or any of $$\alpha_j, \beta_j$$ for $$0 \leq j < i$$.
Otherwise, $$\beta_i$$ is $$v_0 = v_0$$.

Given two sequences $$t = \langle (\alpha_i, \beta_i) : i < n \rangle$$ and $$t' = \langle (\alpha_i, \beta_i) : i < m \rangle$$, we say that $$t'$$ extends $$t$$ iff $$n \leq m$$ and for each $$i < n$$, $$\alpha_i = \alpha_i'$$. Moreover, if $$t' \neq t$$, we say that $$t'$$ properly extends $$t$$.

Claim 6. For every sequence $$t \in T$$, there is a $$t' \in T$$ such that $$t'$$ properly extends $$t$$.

Claim 7. There is an infinite sequence $$\langle (\alpha_i, \beta_i) : i \in \mathbb{N} \rangle$$ such that for every $$n \in \mathbb{N}$$ we have $$\langle (\alpha_i, \beta_i) : i \leq n \rangle \in T$$.

Let $$\bar\Gamma = \{ \alpha_i : i \in \mathbb{N} \} \cup \{ \beta_i : i \in \mathbb{N} \}$$.

Claim 8. All the properties in II) hold for $$\bar\Gamma$$.

We define the first structure $$\mathfrak{A}_0$$:

1. Let its domain $$|\mathfrak{A}_0|$$ be the set of all terms of $$\mathcal{L}_{\bar C}$$.
2. For any constant $$c$$ in $$\mathcal{L}_{\bar C}$$, $c^{\mathfrak{A}_0} = c$
3. For each $$n$$-ary relation symbol $$R$$, define $$R^{\mathfrak{A}_0} \subseteq |\mathfrak{A}_0|^n$$ as $(t_1, \dots, t_n) \in R^{\mathfrak{A}_0} \iff R t_1 \cdots t_n \in \bar\Gamma$
4. For each $$n$$-ary function symbol $$f$$, define $$f^{\mathfrak{A}_0} : |\mathfrak{A}_0|^n \to |\mathfrak{A}_0|$$ as $f^{\mathfrak{A}_0}(t_1, \dots, t_n) = f t_1 \cdots t_n$

And we define the assignment of variables $$s : V \to |\mathfrak{A}_0|$$ as $s(x) = x$ for any variable $$x$$, that is, a term of $$\mathcal{L}_{\bar C}$$.

Furthermore, we define a binary relation $$E$$ on $$|\mathfrak{A}_0|$$ by $t E t' \iff t = t' \in \bar\Gamma$

For any formula $$\varphi$$ of $$\mathcal{L}_{\bar C}$$, let $$\varphi^*$$ denote the result of replacing every occurrence of $$=$$ by $$E$$.

Claim 9. For any formula $$\varphi$$ of $$\mathcal{L}_{\bar C}$$, $\models_{\mathfrak{A}_0} \varphi^*[s] \iff \varphi \in \bar\Gamma$

Claim 10. The relation $$E$$ is an equivalence relation (i.e., $$E$$ is reflexive, symmetric and transitive).

Since $$E$$ is an equivalence relation, we can denote the equivalence class of any term $$t$$ by $$[t]_E$$: $[t]_E = \{ t' : t' \in \text{Term}_{\mathcal{L}_{\bar C}} \land t E t'\}$

We define the quotient structure $$\mathfrak{A} = \mathfrak{A}_0 / E$$ as follows:

1. Let its domain $$|\mathfrak{A}|$$ be $$\{ [t]_E : t \in \text{Term}_{\bar C} \}$$.
2. For any constant $$c$$ in $$\mathcal{L}_{\bar C}$$, $c^\mathfrak{A} = [c]_E$
3. For each $$n$$-ary relation symbol $$R$$, define $$R^{\mathfrak{A}_0} \subseteq |\mathfrak{A}_0|^n$$ as $([t_1]_E, \dots, [t_n]_E) \in R^\mathfrak{A} \iff (t_1, \dots, t_n) \in R^{\mathfrak{A}_0}$
4. For each $$n$$-ary function symbol $$f$$, define $$f^{\mathfrak{A}_0} : |\mathfrak{A}_0|^n \to |\mathfrak{A}_0|$$ as $f^\mathfrak{A}([t_1]_E, \dots, [t_n]_E) = [f^{\mathfrak{A}_0}(t_1, \dots\, t_n)]_E$

And we define the assignment of variables $$s : V \to |\mathfrak{A}|$$ as $s(x) = [x]_E$

Claim 11. The above relations $$R^{\mathfrak{A}_0}$$ and functions $$f^{\mathfrak{A}_0}$$ are well-defined.

Define the homomorphism $$h$$ of $$\mathfrak{A}_0$$ onto $$\mathfrak{A}=\mathfrak{A}_0/E$$: $h(t) = [t]_E$

Claim 12. The $$\mathcal{L}_{\bar C}$$-structure $$\mathfrak{A}$$ satisfies every member of $$\bar\Gamma$$ with $$h \circ s$$.

Proof. (Use the Homomorphism Theorem.)

Finally, we restrict the structure $$\mathfrak{A}$$ to the original language $$\mathcal{L}$$.

Claim 13. The $$\mathcal{L}$$-structure $$\mathfrak{A}$$ satisfies every member of $$\Gamma$$ with $$h \circ s$$.