Principal ultrafilter
= Principal ultrafilter
{wiki=Ultrafilter#Principal_ultrafilters}
The principal ultrafilter at $x\in X$ is $\{A\subseteq X:x\in A\}$. An ultraproduct by a principal ultrafilter is isomorphic to the factor indexed by $x$.
= Principal ultrafilter
{wiki=Ultrafilter#Principal_ultrafilters}
The principal ultrafilter at $x\in X$ is $\{A\subseteq X:x\in A\}$. An ultraproduct by a principal ultrafilter is isomorphic to the factor indexed by $x$.