# nLab A-infinity-algebra

Contents

### Context

#### Higher algebra

higher algebra

universal algebra

# Contents

## Idea

An $A_\infty$-algebra is a monoid internal to a homotopical category such that the associativity law holds not as an equation, but only up to higher coherent homotopy.

## Definition

###### Definition

An $A_\infty$-algebra is an algebra over an operad over an A-∞ operad.

## Realizations

### In chain complexes

Let here $\mathcal{E}$ be the category of chain complexes $\mathcal{Ch}_\bullet$. Notice that often in the literature this choice of $\mathcal{E}$ is regarded as default and silently assumed.

An $A_\infty$-algebra in chain complexes is concretely the following data.

A chain $A_\infty$-algebra is the structure of a degree 1 coderivation

$D : T^c V \to T^c V$

on the reduced tensor coalgebra $T^c V = \oplus_{n\geq 1} V^{\otimes n}$ (with the standard noncocommutative coproduct, see differential graded Hopf algebra) over a graded vector space $V$ such that

$D^2 = 0 \,.$

Coderivations on free coalgebras are entirely determined by their “value on cogenerators”, which allows one to decompose $D$ as a sum:

$D = D_1 + D_2 + D_3 + \cdots$

with each $D_k$ specified entirely by its action

$D_k : V^{\otimes k} \to V \,.$

which is a map of degree $2-k$ (or can be alternatively understood as a map $D_k : (V[1])^{\otimes k}\to V[1]$ of degree $1$).

Then:

• $D_1 : V\to V$ is the differential with $D_1^2 = 0$;

• $D_2 : V^{\otimes 2} \to V$ is the product in the algebra;

• $D_3 : V^{\otimes 3} \to V$ is the associator which measures the failure of $D_2$ to be associative;

• $D_4 : V^{\otimes 4} \to V$ is the pentagonator (or so) which measures the failure of $D_3$ to satisfy the pentagon identity;

• and so on.

One can also allow $D_0$, in which case one talks about weak $A_\infty$-algebras, which are less understood.

There is a resolution of the operad $\mathrm{Ass}$ of associative algebras (as operad on the category of chain complexes) which is called the $A_\infty$-operad; the algebras over the $A_\infty$-operad are precisely the $A_\infty$-algebras.

A morphism of $A_\infty$-algebras $f : A\to B$ is a collection $\lbrace f_n\rbrace_{n\geq 1}$ of maps

$f_n : (A[1])^{\otimes n}\to B[1]$

of degree $0$ satisfying

$\sum_{0\leq i+j\leq n} f_{i+j+1}\circ(1^{\otimes i}\otimes D_{n-i-j}\otimes 1^{\otimes j}) = \sum_{i_1+\ldots+i_r=n} D_r\circ (f_{i_1}\otimes\ldots f_{i_r}).$

For example, $f_1\circ D_1 = D_1\circ f_1$.

#### Rectification

###### Theorem

If $A$ is a dg-algebra, regarded as a strictly associative $A_\infty$-algebra, its chain cohomology $H^\bullet(A)$, regarded as a chain complex with trivial differentials, naturally carries the structure of an $A_\infty$-algebra, unique up to isomorphism, and is weakly equivalent to $A$ as an $A_\infty$-algebra.

More details are at Kadeishvili's theorem.

###### Remark

This theorem provides a large supply of examples of $A_\infty$-algebras: there is a natural $A_\infty$-algebra structure on all cohomologies such as

etc.

### In Topological space

An $A_\infty$-algebra in Top is also called an A-∞ space .

#### Examples

Every loop space is canonically an A-∞ space. (See there for details.)

#### Rectification

###### Theorem

Every $A_\infty$-space is weakly homotopy equivalent to a topological monoid.

This is a classical result by (Stasheff, BoardmanVogt). It follows also as a special case of the more general result on rectification in a model structure on algebras over an operad (see there).

### In spectra

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

algebraic deformation quantization

dimensionclassical field theoryLagrangian BV quantum field theoryfactorization algebra of observables
general $n$P-n algebraBD-n algebra?E-n algebra
$n = 0$Poisson 0-algebraBD-0 algebra? = BD algebraE-0 algebra? = pointed space
$n = 1$P-1 algebra = Poisson algebraBD-1 algebra?E-1 algebra? = A-∞ algebra

## References

A survey of $A_\infty$-algebras in chain complexes is in

• Bernhard Keller, A brief introduction to $A_\infty$-algebras (pdf)

Classical articles on $A_\infty$-algebra in topological spaces are

• Jim Stasheff, Homotopy associativity of H-spaces I , Trans. Amer. Math. Soc. 108 (1963), p. 275-292.

A brief survey is in section 1.8 of

• Martin Markl, Steve Shnider, James D. Stasheff, Operads in algebra, topology and physics, Math. Surveys and Monographs 96, Amer. Math. Soc. 2002.

The 1986 thesis of Alain Prouté explores the possibility of obtaining analogues of minimal models for $A_\infty$ algebras. It was published in TAC much later.

Last revised on February 17, 2017 at 07:20:31. See the history of this page for a list of all contributions to it.