Contents

Definition

An $A_\infty$-space is a homotopy type $X$ that is equipped with the structure of a monoid up to coherent higher homotopy:

that means it is equipped with

1. a binary product operation $\cdot \colon X \times X \to X$

2. a choice of associativity homotopy; $\eta_{x,y,z} : (x\cdot y) \cdot z \to x \cdot (y \cdot z)$;

3. a choice of pentagon law homotopy between five such $\eta$s;

4. and so ever on, as controlled by the associahedra.

In short one may say: an $A_\infty$-space is an A-∞ algebra/monoid in an (∞,1)-category in the (∞,1)-category ∞Grpd/Top. See there for more details.

Properties

Relation to H-monoids

If in the definition of an $A_\infty$-space one discards all the higher homotopies and retains only the existence of an associativity-homotopy, then one has the notion of H-monoid?. Put another way, An $A_\infty$-space in the (∞,1)-category ∞Grpd/Top becomes an H-monoid? in the homotopy Ho(Top). And lifting an H-monoid? structure to an $A_\infty$-space structure means lifting a monoid structure through the projection from the (∞,1)-category ∞Grpd/Top to Ho(Top).

Relation to $A_\infty$-categories

The delooping of an $A_\infty$-space is an A-∞ category/(∞,1)-category with a single object. (Beware that in standard literature “$A_\infty$-category” is often, but not necessarily, reserved for a stable (∞,1)-category).

There is an equivalence of (∞,1)-categories between pointed connected A-∞ categories/(∞,1)-categories and $A_\infty$-spaces.

Related concepts

• model structure for dendroidal left fibrations

• H-space, A-n space

• monoid, monoidal groupoid

• k-tuply associative n-groupoid

• A-infinity category

References

$A_\infty$-spaces were introduced by Jim Stasheff as a refinement of an H-group taking into account higher coherences.

• Jim Stasheff, Homotopy associative H-spaces I, II, Trans. Amer. Math. Soc. 108 (1963), 275–312 MR158400