# Definition:Structure for Predicate Logic

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This page is about Structure in the context of Predicate Logic. For other uses, see Structure.

## Definition

Let $\LL_1$ be the language of predicate logic.

A structure $\AA$ for $\LL_1$ comprises:

$(1): \quad$ A non-empty set $A$;
$(2): \quad$ For each function symbol $f$ of arity $n$, a mapping $f_\AA: A^n \to A$;
$(3): \quad$ For each predicate symbol $p$ of arity $n$, a mapping $p_\AA: A^n \to \Bbb B$

where $\Bbb B$ denotes the set of truth values.

$A$ is called the underlying set of $\AA$.

$f_\AA$ and $p_\AA$ are called the interpretations of $f$ and $p$ in $\AA$, respectively.

We remark that function symbols of arity $0$ are interpreted as constants in $A$.

Also, the predicate symbols may be interpreted as relations via their characteristic functions.

### Formal Semantics

#### Formal Semantics for Sentences

The structures for $\mathcal L_1$ can be interpreted as a formal semantics for $\mathcal L_1$, which we denote by $\mathrm{PL}$.

For the purpose of this formal semantics, we consider only sentences instead of all WFFs.

The structures of $\mathrm{PL}$ are said structures for $\mathcal L_1$.

A sentence $\mathbf A$ is declared ($\mathrm{PL}$-)valid in a structure $\mathcal A$ if and only if:

$\operatorname{val}_{\mathcal A} \left({\mathbf A}\right) = T$

where $\operatorname{val}_{\mathcal A} \left({\mathbf A}\right)$ is the value of $\mathbf A$ in $\mathcal A$.

Symbolically, this can be expressed as:

$\mathcal A \models_{\mathrm{PL}} \mathbf A$

#### Formal Semantics for WFFs

The structures for $\mathcal L_1$ can be interpreted as a formal semantics for $\mathcal L_1$, which we denote by $\mathrm{PL_A}$.

The structures of $\mathrm{PL_A}$ are pairs $\left({\mathcal A, \sigma}\right)$, where:

$\mathcal A$ is a structure for $\mathcal L_1$
$\sigma$ is an assignment for $\mathcal A$

A WFF $\mathbf A$ is declared ($\mathrm{PL_A}$-)valid in a structure $\mathcal A$ if and only if:

$\sigma$ is an assignment for $\mathbf A$
$\mathop{ \operatorname{val}_{\mathcal A} \left({\mathbf A}\right) } \left[{\sigma}\right] = T$

where $\mathop{ \operatorname{val}_{\mathcal A} \left({\mathbf A}\right) } \left[{\sigma}\right]$ is the value of $\mathbf A$ under $\sigma$.

Symbolically, this can be expressed as one of the following:

$\mathcal A, \sigma \models_{\mathrm{PL_A}} \mathbf A$
$\mathcal A \models_{\mathrm{PL_A}} \mathbf A \left[{\sigma}\right]$

## Also known as

A structure for $\LL_1$ is also often called a structure for predicate logic or first-order structure.

The latter formulation is particularly used when the precise vocabulary used for $\LL_1$ is not important.