# Infinitary logic

Infinitary logic is a type of logic which is used in the standard characterizations of several large cardinals, such as weakly compact cardinals and strongly compact cardinals. It also is used in alternate characterizations of other large cardinals such as supercompact cardinals and extendible cardinals.

More formally, an infinitary logic is a formal logic which has strings of infinite length. Generally, there is only one type of infinitary logic which is classically studied: Hilbert-type infinitary logic.

## Hilbert-Type Infinitary Logic

The idea behind the infinitary logic $\mathcal{L}_{\kappa,\lambda}$ is that you can have $\kappa$-many logical additions and logical products in a row, and $\lambda$-many quantifiers in a row. This is called Hilbert-type Infinitary Logic. You can also use $(n+1)$-th order quantifiers in $\mathcal{L}_{\kappa,\lambda}^n$.

### Formal Definition

Let $\kappa$ and $\lambda$ be regular cardinals. Then, $\mathcal{L}_{\kappa,\lambda}$ allows for all first-order finitary assertions (made in $\mathcal{L}_{\omega,\omega}$) along with:

1. For any set of $\mathcal{L}_{\kappa,\lambda}$-formulae $P$ where $|P|<\kappa$, $\bigwedge_{\varphi\in P} \varphi$ and $\bigvee_{\varphi\in P}\varphi$
2. For any set of variables $A$ where $|A|<\kappa$, $\forall_{v\in A}\varphi$ where $\varphi$ is an $\mathcal{L}_{\kappa,\lambda}$-formula
3. For any $\varphi$ which is an $\mathcal{L}_{\kappa,\lambda}$-formula, $\neg\varphi$

There is also $\mathcal{L}_{\infty,\lambda}$, $\mathcal{L}_{\kappa,\infty}$, and $\mathcal{L}_{\infty,\infty}$ where $\infty$ is treated like $\text{ORD}$, allowing for statements of any ordinal length. You can even have $\mathcal{L}_{\infty^+,\infty^+}$ which allows for $\text{ORD}$-length statements.

### Expressiveness

$\mathcal{L}_{\kappa,\kappa}$ is unable to express some $\Pi_1^1$-formulae under ZFC. Contrastively, $\mathcal{L}^1_{\omega,\omega}$ is unable to express $\mathcal{L}_{\omega_1,\omega_1}$, so first-order infinitary logic and second-order finitary logic both have expressiveness advantages. For why, see this question on MathOverflow.