## Lie algebras are groups

November 30, 2015 by Qiaochu Yuan

Once upon a time I imagine people were very happy to think of Lie algebras as “infinitesimal groups,” but presumably when infinitesimals fell out of favor this interpretation did too. In this post I’ll record an observation that can justify thinking of Lie algebras as groups in a strong sense: they are group objects in a certain category which can be interpreted as a category of “infinitesimal spaces.”

Below we work throughout over a field of characteristic zero.

For starters, the universal enveloping algebra functor , which a priori takes values in algebras (it’s left adjoint to the forgetful functor from algebras to Lie algebras), in fact takes values in Hopf algebras. This upgraded functor continues to be a left adjoint, although the forgetful functor is less obvious. Given a Hopf algebra , its primitive elements are those elements satisfying

where is the comultiplication. The primitive elements of a Hopf algebra form a Lie algebra, and this gives a forgetful functor from Hopf algebras to Lie algebras whose left adjoint is the upgraded universal enveloping algebra functor.

The key observation is that this upgraded functor is fully faithful; that is, there is a natural bijection between Lie algebra homomorphisms and Hopf algebra homomorphisms . This is more or less equivalent to the claim that the natural inclusion induces an isomorphism from to the Lie algebra of primitive elements of , which can be proven using the PBW theorem.

Hence Lie algebras embed as a full subcategory of Hopf algebras; that is, they can be thought of as Hopf algebras satisfying certain properties, rather than having extra structure (in the nLab sense). What are these properties? For starters, they are all cocommutative. This is important because cocommutative Hopf algebras are group objects in the category of cocommutative coalgebras (this is *not* true with “cocommutative” dropped!), which in turn can be interpreted as a category of infinitesimal spaces. (For example, this category is cartesian closed, and in particular distributive.)

Hence Lie algebras are group objects in cocommutative coalgebras satisfying some property (for example, “conilpotence”; see Theorem 3.8.1 here).

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on January 14, 2016 at 10:03 am |ChristosNice post! Could you say more about more the phrase: ”… which in turn can be interpreted as a category of infinitesimal spaces” by the end of the post?

on January 14, 2016 at 12:15 pm |Qiaochu YuanOver a field, the category of cocommutative coalgebras is the ind-completion of the category of finite-dimensional cocommutative coalgebras, which is equivalent to the opposite of the category of finite-dimensional commutative algebras.