nLab symmetric ring spectrum



Higher algebra

Stable Homotopy theory



A symmetric ring spectrum is a ring spectrum modeled as a symmetric spectrum. Equivalently this is a monoid object with respect to the symmetric monoidal smash product of spectra, for symmetric spectra. Morever, if we regard symmetric spectra as 𝕊 sym\mathbb{S}_{sym}-modules, as discussed at Model categories of diagram spectra, then this, in turn, is equivalent to a 𝕊 sym\mathbb{S}_{sym}-algebra, where 𝕊 sym\mathbb{S}_{sym} is the standard model of the sphere spectrum as a symmetric spectrum.

There is a model structure for symmetric ring spectra (MMSS 00) under which symmetric ring spectra represent genuine A-infinity rings, and commutative symmetric ring spectra represent genuine E-infinity rings. This fact is one of the key motivation for passing from sequential spectra to the richer model of symmetric spectra (and possibly further to other highly structured spectra such as orthogonal spectra and excisive functors).

Despite all this, the component expression of the structure in a symmetric ring spectrum, in the fashion of functors with smash product, is rather straightforward, and directly analogous to the structure in a dg-algebra: essentially there is for all pairs of indices n 1,n 2n_1, n_2 a pairing between the component spaces of the spectrum in these degrees

E n 1E n 2E n 1+n 2 E_{n_1}\wedge E_{n_2}\longrightarrow E_{n_1 + n_2}

such that this respects the given action of the symmetric groups and of suspensions, and such that it is is associative and unital in the evident way.


Write Top cg *Top_{cg}^{\ast} for the category of pointed compactly generated topological spaces.

One minimal way to state the definition is as follows. We follow conventions as used at model structure on orthogonal spectra.


A commutative symmetric ring spectrum EE is

  1. a sequence of component spaces E nTop cg */E_n \in Top^{\ast/}_{cg} for nn \in \mathbb{N};

  2. a basepoint preserving continuous left action of the symmetric group Σ(n)\Sigma(n) on E nE_n;

  3. for all n 1,n 2n_1,n_2\in \mathbb{N} a multiplication map

    μ n 1,n 2:E n 1E n 2E n 1+n 2 \mu_{n_1,n_2} \;\colon\; E_{n_1} \wedge E_{n_2} \longrightarrow E_{n_1 + n_2}

    (a morphism in Top cg */Top^{\ast/}_{cg})

  4. two unit maps

    ι 0:S 0E 0 \iota_0 \;\colon\; S^0 \longrightarrow E_0
    ι 1:S 1E 1 \iota_1 \;\colon\; S^1 \longrightarrow E_1

such that

  1. μ n 1,n 2\mu_{n_1,n_2} intertwines the Σ(n 1)×Σ(n 2)\Sigma(n_1) \times \Sigma(n_2)-action;

  2. (associativity) for all n 1,n 2,n 3n_1, n_2, n_3 \in \mathbb{N} the following diagram commutes (where we are notationally suppressing the associators of (Top cg */,,S 0)(Top^{\ast/}_{cg}, \wedge, S^0))

    E n 1E n 2E n 3 idμ n 2,n 3 E n 1E n 2+n 3 μ n 1,n 2id μ n 1,n 2+n 3 E n 1+n 2E n 3 μ n 1+n 2,n 3 E n 1+n 2+n 3; \array{ E_{n_1} \wedge E_{n_2} \wedge E_{n_3} &\overset{id \wedge \mu_{n_2,n_3}}{\longrightarrow}& E_{n_1} \wedge E_{n_2 + n_3} \\ {}^{\mathllap{\mu_{n_1,n_2}\wedge id }}\downarrow && \downarrow^{\mathrlap{\mu_{n_1, n_2 + n_3}}} \\ E_{n_1 + n_2} \wedge E_{n_3} &\underset{\mu_{n_1 + n_2, n_3}}{\longrightarrow}& E_{n_1 + n_2 + n_3} } \,;
  3. (unitality) for all nn \in \mathbb{N} the following diagram commutes

    S 0E n ι 0id E 0E n E n Top cg */ μ 0,n E n \array{ S^0 \wedge E_n &\overset{\iota_0 \wedge id}{\longrightarrow}& E_0 \wedge E_n \\ &{}_{\mathllap{\ell^{Top^{\ast/}_{cg}}_{E_n}}}\searrow& \downarrow^{\mathrlap{\mu_{0,n}}} \\ && E_n }


    E nS 0 idι 0 E nE 0 r E n Top cg */ μ n,0 E n \array{ E_n \wedge S^0 &\overset{id \wedge \iota_0 }{\longrightarrow}& E_n \wedge E_0 \\ &{}_{\mathllap{r^{Top^{\ast/}_{cg}}_{E_n}}}\searrow& \downarrow^{\mathrlap{\mu_{n,0}}} \\ && E_n }
  4. (commutativity) for all n 1,n 2n_1, n_2 \in \mathbb{N} the following diagram commutes

    E n 1E n 2 τ E n 1,E n 2 Top cg */ E n 2E n 1 μ n 1,n 2 μ n 2,n 1 E n 1+n 2 χ n 1,n 2 E n 2+n 1, \array{ E_{n_1} \wedge E_{n_2} &\overset{\tau^{Top^{\ast/}_{cg}}_{E_{n_1}, E_{n_2}}}{\longrightarrow}& E_{n_2} \wedge E_{n_1} \\ {}^{\mathllap{\mu_{n_1,n_2}}}\downarrow && \downarrow^{\mathrlap{\mu_{n_2,n_1}}} \\ E_{n_1 + n_2} &\underset{\chi_{n_1,n_2}}{\longrightarrow}& E_{n_2 + n_1} } \,,

where χ n 1,n 2Σ(n 1+n 2)\chi_{n_1,n_2} \in \Sigma(n_1 + n_2) denotes the permutation which shuffles the first n 1n_1 elements past the last n 2n_2 elements.

A homomorphism of symmetric commutative ring spectra f:EEf \colon E \longrightarrow E' is a sequence f n:E nE nf_n \;\colon\; E_n \longrightarrow E'_n of Σ(n)\Sigma(n)-equivariant pointed continuous functions such that the following diagrams commute for all n 1,n 2n_1, n_2 \in \mathbb{N}

E n 1E n 2 f n 1f n 2 E n 1E n 2 μ n 1,n 2 μ n 2,n 1 E n 1+n 2 χ n 1,n 2 E n 2+n 1 \array{ E_{n_1} \wedge E_{n_2} &\overset{f_{n_1} \wedge f_{n_2}}{\longrightarrow}& E'_{n_1} \wedge E'_{n_2} \\ {}^{\mathllap{\mu_{n_1,n_2}}}\downarrow && \downarrow^{\mu_{n_2,n_1}} \\ E_{n_1 + n_2} &\underset{\chi_{n_1, n_2}}{\longrightarrow}& E_{n_2 + n_1} }

and f 0ι 0=ι 0f_0 \circ \iota_0 = \iota_0 and f 1ι 1=ι 1f_1\circ \iota_1 = \iota_1.


CRing(SymSpec(Top cg)) CRing(SymSpec(Top_{cg}))

for the resulting category of symmetric commutative ring spectra.

There is an analogous definition of orthogonal ring spectrum and we write

CRing(OrthSpec(Top cg)) CRing(OrthSpec(Top_{cg}))

for the category that these form.

(Schwede 12, def. 1.3)


The structure of a symmetric ring spectrum on KO is discussed in

  • Michael Joachim, A symmetric ring spectrum representing

    KOKO-theory_, Topology 40 (2001) 299-308

Last revised on June 15, 2017 at 13:38:18. See the history of this page for a list of all contributions to it.