# Symmetric spectrum

In algebraic topology, a symmetric spectrum X is a spectrum of pointed simplicial sets that comes with an action of the symmetric group ${\displaystyle \Sigma _{n}}$ on ${\displaystyle X_{n}}$ such that the composition of structure maps
${\displaystyle S^{1}\wedge \dots \wedge S^{1}\wedge X_{n}\to S^{1}\wedge \dots \wedge S^{1}\wedge X_{n+1}\to \dots \to S^{1}\wedge X_{n+p-1}\to X_{n+p}}$
is equivariant with respect to ${\displaystyle \Sigma _{p}\times \Sigma _{n}}$. A morphism between symmetric spectra is a morphism of spectra that is equivariant with respect to the actions of symmetric groups.
The technical advantage of the category ${\displaystyle {\mathcal {S}}p^{\Sigma }}$ of symmetric spectra is that it has a closed symmetric monoidal structure (with respect to smash product). It is also a simplicial model category. A symmetric ring spectrum is a monoid in ${\displaystyle {\mathcal {S}}p^{\Sigma }}$; if the monoid is commutative, it's a commutative ring spectrum. The possibility of this definition of "ring spectrum" was one of motivations behind the category.