# Definition:Tautology/Formal Semantics/Predicate Logic

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## Definition

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 $\models_{\mathrm{PL_A}} \phi$.

## Sources

- 2009: Kenneth Kunen:
*The Foundations of Mathematics*... (previous) ... (next): $\mathrm{II}.8$ Further Semantic Notions: Definition $\mathrm{II.8.1}$