# nLab abelian infinity-group

Contents

group theory

### Cohomology and Extensions

#### Higher algebra

higher algebra

universal algebra

# Contents

## Idea

An ordinary group is either an abelian group or not. For an ∞-group there is an infinite tower of notions ranging from completely general non-abelian ∞-groups, over braided $\infty$-groups, sylleptic $\infty$-groups …, to ever more abelian groups By an abelian ∞-group (not an established term) one may want to mean an ∞-group which is maximally abelian, in this sense.

Technically, the level of abelianness may be encoded (see at May recognition theorem) by the $E_n$-operads as $n$ ranges from 1 to $\infty$: On the non-abelian end, a general ∞-group is equivalently a groupal algebra over $E_1$, also known as the associative operad, hence is a groupal A-∞ algebra; while at the abelian end a groupal $E_\infty$-space is an infinite loop space or connective spectrum. See also the periodic table of $k$-tuply monoidal $n$-groupoids.

Notice that referring to connective spectra as “abelian $\infty$-groups” (which is not standard) matches the established terminology for non-abelian cohomology (which is standard): The qualifier “non-abelian” in non-abelian cohomology is in contrast to Whitehead-generalized cohomology theories which are represented by spectra.

In a more restrictive sense one may say that plain abelian cohomology is just ordinary cohomology theory, subsuming only those Whitehead-generalized cohomology theories which are represented specifically by Eilenberg-MacLane spectra. Under the Dold-Kan correspondence these are equivalently chain complexes of abelian groups. One may think of these as being yet more commutative than general spectra and might want to reserve the term “abelian $\infty$-group” for them.

## Proposition

### Relation to commutative $\infty$-rings

###### Definition

Write

$gl_1 \; \colon \; CRing_\infty \to AbGrp_\infty$

for the (∞,1)-functor which sends a commutative ∞-ring to its ∞-group of units.

###### Definition

The ∞-group of units (∞,1)-functor of def. is a right-adjoint (∞,1)-functor (or at least a right adjoint on homotopy categories)

$CRing_\infty \stackrel{\overset{\mathbb{S}[-]}{\leftarrow}}{\underset{gl_1}{\to}} AbGrp_\infty \,.$

This is (ABGHR 08, theorem 2.1).

## Examples

A-∞ operadA-∞ algebra∞-groupA-∞ space, e.g. loop spaceloop space object
E-k operadE-k algebrak-monoidal ∞-groupiterated loop spaceiterated loop space object
E-∞ operadE-∞ algebraabelian ∞-groupE-∞ space, if grouplike: infinite loop space $\simeq$ ∞-spaceinfinite loop space object
$\simeq$ connective spectrum$\simeq$ connective spectrum object
stabilizationspectrumspectrum object

## References

General discussion is in section 5 of

Discussion in the context of E-∞ rings and twisted cohomology is in

Last revised on June 25, 2022 at 19:10:10. See the history of this page for a list of all contributions to it.