# Definition:Elementary Embedding

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## Definition

Let $\mathcal{M}$ and $\mathcal{N}$ be $\mathcal{L}$-structures with universes $M$ and $N$ respectively.

An $\mathcal{L}$-embedding $j:\mathcal{M}\to \mathcal{N}$ is an elementary embedding if and only if it preserves truth; that is:

$\mathcal{M} \models \phi(a_1,\dots, a_n) \iff \mathcal{N} \models \phi(j(a_1),\dots, j(a_n))$

holds for all $n\in\mathbb N$, all $\mathcal{L}$-formulas $\phi$ with $n$ free variables, and for all $a_1,\dots,a_n \in M$.

### Partial Elementary Embedding

Let $\mathcal{M}$ and $\mathcal{N}$ be $\mathcal{L}$-structures with universes $M$ and $N$ respectively.

Let $A \subseteq M$ be a subsets of $M$.

$j: A \to \mathcal{N}$ is a partial elementary embedding if and only if it is a partial $\mathcal{L}$-embedding which preserves truth for elements of $A$; that is:

$\mathcal{M} \models \phi(a_1,\dots, a_n) \iff \mathcal{N} \models \phi(j(a_1),\dots, j(a_n))$

holds for all $n \in \N$, all $\mathcal{L}$-formulas $\phi$ with $n$ free variables, and for all $a_1,\dots,a_n \in A$.