Chern-Simons Lie-3-Algebra Inside Derivations of String Lie-2-Algebra
Posted by Urs Schreiber
The -Category Café started with a discussion of the Lie 3-group underlying 11-dimensional supergravity #. In a followup #, I discussed a semistrict Lie 3-algebra with the property that 3-connections taking values in it are Chern-Simons 3-forms with values in , giving the local gauge structure of heterotic string backgrounds.
At that time I guessed that is in fact equivalent to the Lie-3-algebra of inner derivations of the Lie-2-algebra, using the fact that there is only a 1-parameter family of possible Lie 3-algebra structures on the underlying 3-vector space.
It would be quite nice if this were indeed true.
While I still have no full proof that is equivalent (tri-equivalent, if you like) to , I have now checked at least one half of this statement:
there is a morphism and one going the other way such that the composition is the identity on :
So at least sits inside :
The details can be found here:
Chern-Simons and Lie-3-algebras
This (rather unpleasant) computation is a generalization of that in the last section of From Loop Groups to 2-Groups, which shows the equivalence .
I take this as further indication # that the structure 3-group of -Chern-Simons theory is (a subgroup of) .
The above notes make use of some previous computations.
The inner automorphism 3-group of any strict Lie 2-group is computed in the first part of these notes on non fake flat surface transport (discussed here). The FDA description of the corresponding Lie 3-algebra is derived in the second part, in terms of the 2- and 3-form curvature of a 2-connection with values in .
(This is a general mechanism: the equations defining the graded differential of the FDA that describes some Lie -algebra are precisely the (Bianchi- and other) identities satisfied by a connection with values in that Lie -algebra.)
The details of the FDA description of and , together with my notational conventions, can be found in the fda laboratory, example 10 and 12, respectively.
Introduction
For any Lie algebra , there is a semistrict Lie-3-algebra such that 3-connections (i.e. algebroid morphisms from the pair algebroid of to the 3-algebroid ) are given by a -valued 1-form , its curvature 2-form and its Chern-Simons 3-form on .
Another Lie 3-algebra canonically associated to is obtained as follows: The semistrict Baez-Crans Lie 2-algebra is equivalent to the strict Lie 2-algebra For any strict Lie 2-algebra , the Lie-3-algebra of its inner derivations is characterized by the fact that 3-connections are given by an -valued 1-form , an -valued 2-form such that with and we have and Had we in addition required that , then this would characterize itself. For and we write It is known that Here we are after a generalization of this equivalence when passing from to . We fall short of actually proving an equivalence. Instead we construct a morphism and a morphism such that is the identity on . This is done for .
We will work throughout in terms of the Koszul dual description of semistrict Lie--algebras. Every Lie--algebra is encoded in a free differential graded algebra, and morphisms of Lie--algebras are given by maps between FDAs that are at the same time chain maps and algebra homomorphisms.
draft commentary on fda lab
For Lie algebras and their generalizations, the graded symmetric algebra version of cochains with field coefficients is actually the Hom dual of the graded symmetric coalgebra version. The advantage of the algebra version, in addition to familiarity, is that properties of a derivation need only be checked on generators.
The graded symmetric coalgebra is naturally a subcoalgebra of the tensor space i.e. is spanned by graded symmetric tensors for where I use ratrher than to emphasize the coalgebra aspect. e.g. notice: no factor of needed.
Instead of checking a coderivation on cogenerators we check on elements with image in .
So let be a Lie algebra. Define a codifferential with regarded as of degree 1 by
and extend as a coderivation that means
where the sum is over and and are omitted in the if they are bracketed.
to check , we need only check it on
where it is readily seen to correspond to the Jacobi identity.
Now if is a dg Lie algebra, i.e. with a differential derivation then regrade before taking i.e. , where denotes the shift or if you prefer - I can never remember this notation.
Define by
and
Now follows from jacobi and being a derivation and .
Modulo an issue about one of the axioms for a Lie crossed module the latter is a dg Lie algebra.
Turning to example 7 of Urs’ fda lab, the additonal term is
The additional verification for follows from the invariance of
Now Example 8 has a surprise there is a term in other words, we have an algebra with for !