# Definition:Tautology/Formal Semantics

## Definition

Let $\mathcal L$ be a logical language.

Let $\mathscr M$ be a formal semantics for $\mathcal L$.

A logical formula $\phi$ of $\mathcal L$ is a **tautology for $\mathscr M$** if and only if:

That $\phi$ is a **tautology for $\mathscr M$** can be denoted as:

- $\models_{\mathscr M} \phi$

## Examples

### Tautology for Boolean Interpretations

Let $\mathbf A$ be a WFF of propositional logic.

Then $\mathbf A$ is called a **tautology (for boolean interpretations)** if and only if:

- $\map v {\mathbf A} = T$

for every boolean interpretation $v$ of $\mathbf A$.

That $\mathbf A$ is a **tautology** may be denoted as:

- $\models_{\mathrm {BI} } \mathbf A$

### Tautology for Predicate Logic

Let $\mathbf A$ be a WFF of predicate logic.

Then $\mathbf A$ is a **tautology** if and only if:

- $\mathcal A, \sigma \models_{\mathrm{PL_A}} \mathbf A$

for every structure $\mathcal A$ and assignment $\sigma$.

That $\mathbf A$ is a **tautology** can be denoted as:

- $\models_{\mathrm{PL_A}} \mathbf A$

## Also known as

In this context, **tautologies** are also referred to as **(logically) valid formulas**.

However, on $\mathsf{Pr} \infty \mathsf{fWiki}$, this can easily be confused with a formula that is valid in a single structure, and is therefore discouraged.

## Also denoted as

When the formal semantics under discussion is clear from the context, $\models \phi$ is a common shorthand for $\models_{\mathscr M} \phi$.

## Also see

- Definition:Top (Logic), a symbol often used to represent
**tautologies**in logical languages. - Definition:Contradiction
- Definition:Contingent Statement

## Sources

- 1996: H. Jerome Keisler and Joel Robbin:
*Mathematical Logic and Computability*... (previous) ... (next): $\S 1.6$: Truth Tables and Tautologies