# nLab L-infinity-algebra

## Examples

### $\infty$-Lie algebras

#### Higher algebra

higher algebra

universal algebra

## Theorems

#### Rational homotopy theory

and

rational homotopy theory

# Contents

## Idea

$L_\infty$-algebras (or strong homotopy Lie algebras) are a higher generalization (a “vertical categorification”) of Lie algebras: in an $L_\infty$-algebra the Jacobi identity is allowed to hold (only) up to higher coherent homotopy.

An $L_\infty$-algebra that is concentrated in lowest degree is an ordinary Lie algebra. If it is concentrated in the lowest two degrees is is a Lie 2-algebra, etc.

From another perspective: an $L_\infty$-algebra is a Lie ∞-algebroid with a single object.

$L_\infty$-algebras are infinitesimal approximations of smooth ∞-groups in analogy to how an ordinary Lie algebra is an infinitesimal approximation of a Lie group. Under Lie integration every $L_\infty$-algebra $\mathfrak{g}$ “exponentiates” to a smooth ∞-group $\Omega \exp(\mathfrak{g})$.

## Definition

### Abstract definition in terms of algebras over an operad

An $L_\infty$-algebra is an algebra over an operad in the category of chain complexes over the L-∞ operad.

In the following we spell out in detail what this means in components.

### Definition in terms of higher brackets

For $V$ a graded vector space, for $v_i \in V_{\vert v_i\vert}$ homogenously graded elements, and for $\sigma$ a permutation of $n$ elements, write $\chi(\sigma, v_1, \cdots, v_n)\in \{-1,+1\}$ for the product of the signature of the permutation with a factor of $(-1)^{\vert v_i \vert \vert v_j \vert}$ for each interchange of neighbours $(\cdots v_i,v_j, \cdots )$ to $(\cdots v_j,v_i, \cdots )$ involved in the permutation.

###### Definition

An $L_\infty$-algebra is

1. a graded vector space $V$;

2. for each $n \in \mathbb{N}$ a multilinear map called the $n$-ary bracket

$l_n(\cdots) \coloneqq [-,-, \cdots, -] \colon V^{\wedge n} \to V$

of degree $n-2$

such that

1. each $l_n$ is graded antisymmetric, in that for every permutation $\sigma$ and homogeneously graded elements $v_i \in V_{\vert v_i \vert}$ then

$l_n(v_{\sigma(1)}, v_{\sigma(2)},\cdots ,v_{\sigma(n)}) = \chi(\sigma,v_1,\cdots, v_n) \cdot l_n(v_1, v_2, \cdots v_n)$
2. the generalized Jacobi identity holds:

(1)$\sum_{i+j = n+1} \sum_{\sigma \in UnShuff(i,n-i)} \chi(\sigma,v_1, \cdots, v_m) (-1)^{i(j-1)} l_{j} \left( l_i \left( v_{\sigma(1)}, \cdots, v_{\sigma(i)} \right), v_{\sigma(i+1)} , \cdots , v_{\sigma(n)} \right) = 0 \,,$

for all $n$, all and homogeneously graded elements $v_i \in V_i$ (here the inner sum runs over all $(i,j)$-unshuffles $\sigma$).

There are various different conventions on the gradings possible, which lead to similar formulas with different signs.

###### Example

In lowest degrees the generalized Jacobi identity says that

1. for $n = 1$: the unary map $\partial \coloneqq l_1$ squares to 0:

$\partial (\partial(v_1)) = 0$

1: for $n = 2$: the unary map $\partial$ is a graded derivation of the binary map

$- [\partial v_1, v_2] - (-1)^{\vert v_1 \vert \vert v_2 \vert} [\partial v_2, v_1] + \partial [v_1, v_2] = 0$

hence

$\partial [v_1, v_2] = [\partial v_1, v_2] + (-1)^{\vert v_1 \vert} [v_1, \partial v_2] \,.$
###### Example

When all higher brackets vanish, $l_{k \gt 2}= 0$ then for $n = 3$:

$[[v_1,v_2],v_3] + (-1)^{\vert v_1 \vert (\vert v_2 \vert + \vert v_3 \vert)} [[v_2,v_3],v_1] + (-1)^{\vert v_2 \vert (\vert v_1 \vert + \vert v_3 \vert)} [[v_1,v_3],v_2] = 0$

this is the graded Jacobi identity. So in this case the $L_\infty$-algebra is equivalently a dg-Lie algebra.

###### Example

When $l_3$ is possibly non-vanishing, then on elements $x_i$ on which $\partial = l_1$ vanishes then the generalized Jacobi identity for $n = 3$ gives

$[[v_1,v_2],v_3] + (-1)^{\vert v_1 \vert (\vert v_2 \vert + \vert v_3 \vert)} [[v_2,v_3],v_1] + (-1)^{\vert v_2 \vert (\vert v_1 \vert + \vert v_3 \vert)} [[v_1,v_3],v_2] = - \partial [v_1, v_2, v_3] \,.$

This shows that the Jacobi identity holds up to an “exact” term, hence up to homotopy.

### Reformulation in terms of semifree differential coalgebra

A little later it was realized that the above huge sum expressions above just expresses the fact that the differential $D$ in a semifree dg-coalgebra squares to 0, $D^2 = 0$:

An $L_\infty$-algebra is

• an $\mathbb{N}_+$-graded vector space $\mathfrak{g}$;

• equipped with a differential $D : \vee^\bullet \mathfrak{g} \to \vee^\bullet \mathfrak{g}$ of degree $-1$ on the free graded co-commutative coalgebra over $\mathfrak{g}$ that squares to 0

$D^2 = 0 \,.$

Here the free graded co-commutative co-algebra $\vee^\bullet \mathfrak{g}$ is, as a vector space, the same as the graded Grassmann algebra $\wedge^\bullet \mathfrak{g}$ whose elements we write as

$3 t_1 \vee t_2 + t_3 + t_3 \vee t_4 \vee t_5$

etc (where the $\vee$ is just a funny way to write the wedge $\wedge$, in order to remind us that:…)

but throught of as equipped with the standard coproduct

$\Delta (v_1 \vee v_2 \cdots \vee v_n) \propto \sum_i \pm (v_1 \vee \cdots \vee v_i) \otimes (v_{i+1} \vee \cdots \vee v_n)$

(work out or see the references for the signs and prefacors).

Since this is a free graded co-commutative coalgebra, one can see that any differential

$D : \vee^\bullet \mathfrak{g} \to \vee^\bullet \mathfrak{g}$

on it is fixed by its value “on cogenerators” (a statement that is maybe unfamiliar, but simply the straightforward dual of the more familar statement to which we come below, that differentials on free graded algebras are fixed by their action on generators) which means that we can decompose $D$ as

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

where each $D_i$ acts as $l_i$ when evaluated on a homogeneous element of the form $t_1 \vee \cdots \vee t_n$ and is then uniquely extended to all of $\vee^\bullet \mathfrak{g}$ by extending it as a coderivation on a coalgebra.

For instance $D_2$ acts on homogeneous elements of word lenght 3 as

$D_2(t_1 \vee t_2 \vee t_3) = D_2(t_1, t_2)\vee t_3 \pm permutations \,.$

exercise for the reader: spell this all out more in detail with all the signs and everyrthing. Possibly by looking it up in the references given below.

Using this, one checks that the simple condition that $D$ squares to 0 is precisely equivalent to the infinite tower of generalized Jacobi identities:

$(D^2 = 0) \Leftrightarrow \left( \forall n : \sum_{i+j = n} \sum_{shuffles \sigma} \pm l_i (l_j (v_{\sigma(1)}, \cdots, v_{\sigma(j)} ) , v_{\sigma(j+1)} , \cdots , v_{\sigma(n)} ) = 0 \right) \,.$

So in conclusion we have:

An $L_\infty$-algebra is a dg-coalgebra whose underlying coalgebra is cofree and concentrated in negative degree.

### Reformulation in terms of semifree differential algebra

The reformulation of an $L_\infty$-algebra as simply a semi-co-free graded-co-commutative coalgebra $(\vee^\bullet \mathfrak{g}, D)$ is a useful repackaging of the original definition, but the coalgebraic aspect tends to be not only unfamiliar, but also a bit inconvenient. At least when the graded vector space $\mathfrak{g}$ is degreewise finite dimensional , we can simply pass to its degreewise dual graded vector space $\mathfrak{g}^*$.

(Fully generally the following works when using not just dg-algebras but pro-objects in dg-algebras, see at model structure for L-infinity algebras – Use of pro-dg-algebras).

Its Grassmann algebra $\wedge^\bullet \mathfrak{g}^*$ is then naturally equipped with an ordinary differential $d = D^*$ which acts on $\omega \in \wedge^\bullet \mathfrak{g}^*$ as

$(d \omega) (t_1 \vee \cdots \vee t_n) = \pm \omega(D(t_1 \vee \cdots \vee t_n)) \,.$

When the grading-dust has settled one finds that with

$\wedge^\bullet \mathfrak{g}^* = k \oplus \mathfrak{g}^*_1 \oplus (\mathfrak{g}^*_1 \wedge \mathfrak{g}^*_1 \oplus \mathfrak{g}^*_2) \oplus \cdots$

with the ground field in degree 0, the degree 1-elements of $\mathfrak{g}^*$ in degree 1, etc, that $d$ is of degree +1 and of course squares to 0

$d^2 = 0 \,.$

This means that we have a semifree dga

$CE(\mathfrak{g}) := (\wedge^\bullet \mathfrak{g}^*, d) \,.$

In the case that $\mathfrak{g}$ happens to be an ordinary Lie algebra, this is the ordinary Chevalley-Eilenberg algebra of this Lie algebra. Hence we should generally call $CE(\mathfrak{g})$ the Chevalley-Eilenberg algebra of the $L_\infty$-algebra $\mathfrak{g}$.

One observes that this construction is bijective: every (degreewise finite dimensional) cochain semifree dga generated in positive degree comes from a (degreewise finite dimensional) $L_\infty$-algebra this way.

This means that we may just as well define a (degreewise finite dimensional) $L_\infty$-algebra as an object in the opposite category of (degreewise finite dimensional) commutative dg-algebras that are semifree dgas and generated in positive degree.

And this turns out to be one of the most useful perspectives on $L_\infty$-algebras.

In particular, if we simply drop the condition that the dg-algebra be generated in positive degree and allow it to be generated in non-negative degree over the algebra in degree 0, then we have the notion of the (Chevalley-Eilenberg algebra of) an L-infinity-algebroid.

#### Details

We discuss in explit detail the computation that shows that an $L_\infty$-algebra structure on $\mathfrak{g}$ is equivalently a dg-algebra-structure on $\wedge^\bullet \mathfrak{g}^*$.

Let $\mathfrak{g}$ be a degreewise finite-dimensional $\mathbb{N}_+$graded vector space equipped with multilinear graded-symmetric maps

$[-,\cdots,-]_k : Sym^k \mathfrak{g} \to \mathfrak{g}$

of degree -1, for each $k \in \mathbb{N}_+$.

Let $\{t_a\}$ be a basis of $\mathfrak{g}$ and $\{t^a\}$ a dual basis of the degreewise dual $\mathfrak{g}^*$. Equip the Grassmann algebra $Sym^\bullet \mathfrak{g}^*$ with a derivation

$d : Sym^\bullet \mathfrak{g}^* \to Sym^\bullet \mathfrak{g}^*$

defined on generators by

$d : t^a \mapsto - \sum_{k = 1}^\infty \frac{1}{k!} [t_{a_1}, \cdots, t_{a_k}]^a_k \, t^{a_1} \wedge \cdots \wedge t^{a_k} \,.$

Here we take $t^a$ to be of the same degree as $t_a$. Therefore this derivation has degree +1.

We compute the square $d^2 = d \circ d$:

\begin{aligned} d d t^a &= d (-1)\sum_{k = 1}^\infty \frac{1}{k!} [t_{a_1}, \cdots, t_{a_k}]^a_k \, t^{a_1} \wedge \cdots \wedge t^{a_k} \\ & = \sum_{k,l = 1}^\infty \frac{1}{(k-1)! l!} [[t_{b_1}, \cdots, t_{b_l}], t_{a_2}, \cdots, t_{a_k}]^a \, t^{b_1} \wedge \cdots \wedge t^{b_l} \wedge t^{a_2} \wedge \cdots \wedge t^{a_{k}} \end{aligned} \,.

Here the wedge product on the right projects the nested bracket onto its graded-symmetric components. This is produced by summing over all permutations $\sigma \in \Sigma_{k+l-1}$ weighted by the Koszul-signature of the permutation:

$\cdots = \sum_{k,l = 1}^\infty \frac{1}{(k+l-1)!} \sum_{\sigma \in \Sigma_{k+l-1}} (-1)^{sgn(\sigma)} \frac{1}{(k-1)! l!} [[t_{b_1}, \cdots, t_{b_l}], t_{a_2}, \cdots, t_{a_k}]^a \, t^{b_1} \wedge \cdots \wedge t^{b_l} \wedge t^{a_2} \wedge \cdots \wedge t^{a_{k}} \,.$

The sum over all permutations decomposes into a sum over the $(l,k-1)$-unshuffles and a sum over permutations that act inside the first $l$ and the last $(k-1)$ indices. By the graded-symmetry of the bracket, the latter do not change the value of the nested bracket. Since there are $(k-1)! l!$ many of them, we get

$\cdots = \sum_{k,l = 1}^\infty \frac{1}{(k+l-1)!} \sum_{\sigma \in Unsh(l,k-1)} (-1)^{sgn(\sigma)} [[t_{a_1}, \cdots, t_{a_l}], t_{a_{l+1}}, \cdots, t_{a_{k+l-1}}] \, t^{a_1} \wedge \cdots \wedge t^{a_{k+l-1}} \,.$

Therefore the condition $d^2 = 0$ is equivalent to the condition

$\sum_{k+l = n-1} \sum_{\sigma \in Unsh(l,k-1)} (-1)^{sgn(\sigma)} [[t_{a_1}, \cdots, t_{a_l}], t_{a_{l+1}}, \cdots, t_{a_{k+l-1}}] = 0$

for all $n \in \mathbb{N}$ and all $\{t_{a_i} \in \mathfrak{g}\}$. This is equation (1) which says that $\{\mathfrak{g}, \{[-,\dots,-]_k\}\}$ is an $L_\infty$-algebra.

### In terms of algebras over an operad

$L_\infty$-algebras are precisely the algebras over an operad of the cofibrant resolution of the Lie operad.

## Examples

### Special cases

• An $L_\infty$-algebra for which $V$ is concentrated in the first $n$ degree is a Lie $n$-algebra (sometimes also: “$L_n$-algebra”).

• An $L_\infty$-algebra for which only the unary operation and the binary bracket are non-trivial is a dg-Lie algebra: a Lie algebra internal to the category of dg-algebras. From the point of view of higher Lie theory this is a strict $L_\infty$-algebra: one for which the Jacobi identity does happen to hold “on the nose”, not just up to nontrivial coherent isomorphisms.

• So in particular

• an $L_\infty$-algebra generated just in degree 1 is an ordinary Lie algebra ;

• an $L_\infty$-algebra generated just in degree 1 and 2 is a Lie 2-algebra ;

• an $L_\infty$-algebra generated just in degree 1, 2 and 3 is a Lie 3-algebra ;

• if $\mathfrak{g}$ is a Lie algebra over $\mathbf{K}$, and $b^{k-1}\mathbb{K}$ is the complex consisting of the field $\mathbb{K}$ in degree $1-k$, then an $L_\infty$-algebra morphism from $\mathfrak{g}$ to $b^{k-1}\mathbb{K}$ is precisely a degree $k$ Lie algebra cocycle.

• The skew-symmetry of the Lie bracket is retained strictly in $L_\infty$-algebras. It is expected that weakening this, too, yields a more general vertical categorification of Lie algebras. For $n=2$ this has been worked out by Dmitry Roytenberg: On weak Lie 2-algebras.

• The horizontal categorification of $L_\infty$-algebras are $L_\infty$-algebroids.

• An $L_\infty$-algebra with only $D_n$ non-vanishing is called an n-Lie algebra – to be distinguished from a Lie $n$-algebra ! However, in large parts of the literature $n$-Lie algebras are considered for which $D_n$ is not of the required homogeneous degree in the grading, or in which no grading is considered in the first place. Such $n$-Lie algebras are not special examples of $L_\infty$-algebras, then. For more see n-Lie algebra.

• An $L_\infty$-alghebra internal to super vector spaces is a super L-∞ algebra.

## Properties

### Relation to dg-Lie algebras

Every dg-Lie algebra is in an evident way an $L_\infty$-algebra. Dg-Lie algebras are precisely those $L_\infty$-algebras for which all $n$-ary brackets for $n \gt 2$ are trivial. These may be thought of as the strict $L_\infty$-algebras: those for which the Jacobi identity holds on the nose and all its possible higher coherences are trivial.

###### Theorem

Let $k$ be a field of characteristic 0 and write $L_\infty Alg_k$ for the category of $L\infty$-algebras over $k$.

Then every object of $L_\infty Alg_k$ is quasi-isomorphic to a dg-Lie algebra.

Moreover, one can find a functorial replacement: there is a functor

$W : L_\infty Alg_k \to L_\infty Alg_k$

such that for each $\mathfrak{g} \in L_\infty Alg_k$

1. $W(\mathfrak{k})$ is a dg-Lie algebra;

2. there is a quasi-isomorphism

$\mathfrak{g} \stackrel{\simeq}{\to} W(\mathfrak{g}) \,.$

This appears for instance as (KrizMay, cor. 1.6).

### Relation to $\infty$-Lie groupoids

In generalization to how a Lie algebra integrates to a Lie group, $L_\infty$-algebras integrate to ∞-Lie groups.

See

Lie integration

and

## References

### General

The original references are:

A discussion in terms of resolutions of the Lie operad is for instance in

A historical survey is

A detailed reference for Lie 2-algebras is:

• John Baez and Alissa Crans, Higher-dimensional algebra VI: Lie 2-algebras, TAC 12, (2004), 492–528. (arXiv)

For more general ‘weak Lie 2-algebras’, see:

Discussion of bilinear invariant polynomials on $L_\infty$-algebras includes

• Yunfeng Jiang, Motivic Milnor fiber of cyclic $L_\infty$-algebras (arXiv:0909.2858)

### $L_\infty$-algebras in physics

The following lists, mainly in chronological order of their discovery, L-∞ algebra structures appearing in physics, notably in supergravity, BV-BRST formalism, string theory, higher Chern-Simons theory/AKSZ sigma-models and local field theory.

#### In supergravity

In their equivalent formal dual guise of Chevalley-Eilenberg algebras (see above), $L_\infty$-algebras of finite type – in fact super L-∞ algebras – appear in pivotal role in the D'Auria-Fré formulation of supergravity at least since

In the supergravity literature these CE-algebras are referred to as “FDA”s. This is short for “free differential algebra”, which is a slight misnomer for what in mathematics are called semifree dgas (or sometimes “quasi-free” dga-s).

The translation of D'Auro-Fré formalism to explicit (super) $L_\infty$-algebra language is made in

Further exposition of this includes

#### In BV-BRST formalism

The understanding that the BV-BRST complex used in physics to model the derived critical locus of the action functionals of gauge theories is mathematically the formal dual Chevalley-Eilenberg algebra of a derived L-∞ algebroid originates around

Discussion in terms of homotopy Lie-Rinehart pairs is due to

• Lars Kjeseth, Homotopy Rinehart cohomology of homotopy Lie-Rinehart pairs, Homology Homotopy Appl. Volume 3, Number 1 (2001), 139-163. (Euclid)

The L-∞ algebroid-structure is also made explicit in (def. 4.1 of v1) of (Sati-Schreiber-Stasheff 09).

#### In string field theory

The first explicit appearance of $L_\infty$-algebras in theoretical physics is the $L_\infty$-algebra structure on the BRST complex of the closed bosonic string found in the context of closed bosonic string field theory in

• Barton Zwiebach, Closed string field theory: Quantum action and the B-V master equation , Nucl.Phys. B390 (1993) 33 (arXiv:hep-th/9206084)

• Jim Stasheff, Closed string field theory, strong homotopy Lie algebras and the operad actions of moduli space Talk given at the Conference on Topics in Geometry and Physics (1992) (arXiv:hep-th/9304061)

Generalization to open-closed bosonic string field theory yields L-∞ algebra interacting with A-∞ algebra:

• Jim Stasheff, Higher homotopy algebras: String field theory and Drinfeld’s quasiHopf algebras, proceedings of International Conference on Differential Geometric Methods in Theoretical Physics, 1991 (spire)

#### In heterotic string theory

Next it was again $L_\infty$-algebras of finite type that drew attention. It was eventually understood that the string structures which embody a refinement of the Green-Schwarz anomaly cancellation mechanism in heterotic string theory have a further smooth refinement as G-structures for the string 2-group, which is the Lie integration of a Lie 2-algebra called the string Lie 2-algebra. This is due to

and the relation to the Green-Schwarz mechanism is made explicit in

This article also observes that an analogous situation appears in dual heterotic string theory with the fivebrane Lie 6-algebra in place of the string Lie 2-algebra.

#### Higher Chern-Simons field theory and AKSZ sigma-models

Ordinary Chern-Simons theory for a simple gauge group is all controled by a Lie algebra 3-cocycle. The generalization of Chern-Simons theory to AKSZ-sigma models was understood to be encoded by symplectic Lie n-algebroids (later re-popularized as “shifted symplectic structures”) in

The globally defined AKSZ action functionals obtained this way were shown in

to be a special case of the higher Lie integration process of

Further exmaples of non-symplectic $L_\infty$-Chern-Simons theory obtained this way include 7-dimensional Chern-Simons theory on string 2-connections:

#### In local prequantum field theory

Infinite-dimensional $L_\infty$-algebras that behaved similar to Poisson bracket Lie algebrasPoisson bracket Lie n-algebras – were noticed

In

these were shown to be the infinitesimal version of the symmetries of prequantum n-bundles as they appear in local prequantum field theory, in higher generalization of how the Poisson bracket is the Lie algebra of the quantomorphism group.

These also encode a homotopy refinement of the Dickey bracket on Noether conserved currents which for Green-Schwarz sigma models reduces to Lie $n$-algebras of BPS charges which refine super Lie algebras such as the M-theory super Lie algebra:

This makes concrete the suggestion that there should be $L_\infty$-algebra refinements of the Dickey bracket of conserved currents in local field theory that was made in

Comprehesive survey and exposition of this situation is in

#### In double field theory

Revised on April 27, 2016 11:39:08 by Marcel Rubió (134.58.253.57)