Let $H$ be a Hopf algebra over a field $k$. What are some nice conditions for when $H$ is isomorphic to $k[G]$ for a finite group $G$?

The co-multiplication structure on the group algebra $k[G]$ is given the by map sending $g$ to $g \otimes g$.

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    $\begingroup$ Related: math.stackexchange.com/questions/2595567 $\endgroup$ – Watson Jun 28 '18 at 20:05
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    $\begingroup$ @Watson: it's not that closely related. The functor from groups to Hopf algebras is fully faithful; you can recover the underlying group from $k[G]$ as the grouplike elements. So the situation is pretty different from if you only have access to the ring structure. $\endgroup$ – Qiaochu Yuan Jun 28 '18 at 22:04
  • $\begingroup$ Your description of the comultiplication is incorrect; what you've described is the comultiplication on the dual Hopf algebra of functions $G \to k$. The comultiplication on $k[G]$ is $\Delta(g) = g \otimes g$. $\endgroup$ – Qiaochu Yuan Jun 28 '18 at 22:08
  • $\begingroup$ My apologies, it's fixed. $\endgroup$ – Dean Young Jun 28 '18 at 23:59

Two straightforward necessary conditions are that $H$ be finite-dimensional and that it be cocommutative. At this point it will be cleaner to work with the dual Hopf algebra $H^{\ast}$, which is finite-dimensional and commutative. Because it is commutative, we can use the language of algebraic geometry: $\text{Spec } H^{\ast}$ makes sense and is a finite group scheme over $k$. Now the question is: when is a finite group scheme the constant group scheme $G$ for $G$ a finite group? This corresponds to $H^{\ast}$ being the Hopf algebra of functions $G \to k$, dual to the group algebra $H \cong k[G]$.

The answer is almost never. If $k$ isn't separably closed, then you can construct interesting nonconstant group schemes from pairs consisting of a finite group $G$ and an action of the absolute Galois group $\text{Gal}(k_s/k)$ on $G$, which are constant iff the action is trivial. These are precisely the finite étale group schemes. If $k$ is separably closed, then every finite étale group scheme over $k$ is constant, but in general there are finite group schemes that are not étale. For example, if $k$ has characteristic $p$, then $k[x]/x^p$ can be equipped with a comultiplication $\Delta(x) = x \otimes 1 + 1 \otimes x$ making $\text{Spec } k[x]/x^p$ a finite nonconstant group scheme called $\alpha_p$. It arises naturally as the kernel of the Frobenius map $\mathbb{G}_a \to \mathbb{G}_a$.

So now let's assume that $k$ is separably closed and has characteristic $0$ (so $k$ is algebraically closed). Finally some good news: in this case every finite group scheme is étale and hence constant. In other words,

If $k$ is an algebraically closed field of characteristic zero, then every finite-dimensional cocommutative Hopf algebra over $k$ is a group algebra.

This is a special case of the Cartier-Kostant-Milnor-Moore theorem, although presumably it has a more elementary proof.


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