Definition:Tautology/Formal Semantics/Predicate Logic

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

Then $\mathbf A$ is a tautology :


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

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