Baer-Schreier-Ulam theorem

Statement

Let $S$ be an infinite set and $\mathrm{S}\mathrm{y}\mathrm{m}(S)$ denote the symmetric group on $S$. For every cardinal $\alpha \le \left|S\right|$, define ${\mathrm{S}}_{\mathrm{y}}(S)$ as the group of all permutations on $S$ that move at most $\alpha$ elements. Then, the normal subgroups of $\mathrm{S}\mathrm{y}\mathrm{m}(S)$ are as follows:

• The trivial subgroup.
• The finitary alternating group: The group of all even finitary permutations.
• The finitary symmetric group: The group of all finitary permutations.
• The subgroups ${\mathrm{S}}_{\mathrm{y}}(S)$ for all ordinals $\alpha \le \left|S\right|$, where ${\mathrm{S}}_{\mathrm{y}}(S)$ is the group of permutations whose support has size equal to the cardinality of any ordinal smaller than $\alpha$.

Related facts

• Finitary alternating group is monolith in symmetric group
• Finitary symmetric group is characteristic in symmetric group