# nLab ring spectrum

Contents

### Context

#### Stable Homotopy theory

stable homotopy theory

Introduction

# Contents

#### Higher algebra

higher algebra

universal algebra

# Contents

## Idea

The terminology (commutative) ring spectrum refers either to a (commutative) monoid in the stable homotopy category regarded as monoidal category via the smash product of spectra, or to the richer structure of a monoid in a model structure for spectra equipped with a symmetric smash product of spectra.

In the first case a ring spectrum is a spectrum equipped with a unit and product operation, which is associative, unital (and commutative) just up to unspecified homotopy, such as in an H-space structure. Accordingly, these might be called “H-ring spectra”, but it is traditional to just call them “ring spectra.” (An H-infinity ring spectrum (Bruner-May-McClure-Steinberger 86) is such an H-ring spectrum equipped with some extra structure modeling extended power operations.)

In the second case the structure is much richer; and in good cases, such as for highly structured spectra, is equivalent to A-∞ ring structure (E-∞ ring structure).

To distinguish the two situations further qualification is being used. Sometimes one says homotopy ring spectrum to explicitly refer to the first case (e.g. Schwede 12, chapter II 4.1) or one says “highly structured ring spectrum” to refer explicitly to the second case. For more on this see at brave new algebra and higher algebra.

Since the concept of spectrum is the refinement of the concept of abelian group to homotopy theory/(∞,1)-category theory. The concept of ring spectrum is the corresponding generalization of the notion of (commutative) ring.

## Definition

For details see Introduction to Stable homotopy theory, Part 1-2 – Structured spectra.

## Properties

### Relation to multiplicative generalized cohomology

Under the Brown representability theorem, the generalized cohomology theory represented by a ring spectrum inherits the structure of a multiplicative cohomology theory.

Conversely via the Brown representability theorem a spectrum representing a multiplicative cohomology theory inherits the structure of (at least) an H-ring spectrum. See there.

## References

The notion of (commutative) homotopy ring spectra, i.e. (commutative) monoids in the stable homotopy category with respect to the smash product of spectra:

Review:

Discussion of connective ring spectra as monoids with respect to a smash product on Gamma-spaces:

• Stefan Schwede, Stable homotopical algebra and $\Gamma$-spaces, Math. Proc. Camb. Phil. Soc. (1999), 126, 329 (pdf)

• Tyler Lawson, Commutative Γ-rings do not model all commutative ring spectra, Homology Homotopy Appl. Volume 11, Number 2 (2009), 189-194. (Euclid)

Discussion of ring spectra as rings with respect to the symmetric smash product of spectra on S-modules includes

A comprehensive account for symmetric spectra is in

and for orthogonal spectra in

An account in terms of (∞,1)-category theory is in section 7.1 of

Discussion of simplicial ring spectra is in