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Visiting professor at the University of Delaware.


Feb
4
revised The structure of symmetric powers of finite-dimensional local rings
added 151 characters in body
Feb
4
asked The structure of symmetric powers of finite-dimensional local rings
Dec
26
comment Enveloping algebras of map algebras as hyperalgebras of algebraic groups
Ok, thanks - that's very helpful! I appreciate it. I'll check out the reference and think about how to lift the Lie algebra statement to a hyperalgebraic statement.
Dec
26
awarded  Civic Duty
Dec
26
comment Enveloping algebras of map algebras as hyperalgebras of algebraic groups
I think $U(\mathfrak g_A)$ sees more than just the underlying vector space of $A$; multiplication in $A$ is part of the Lie bracket in $\mathfrak g_A$, so commutation in $U$ sees the algebra structure of $A$. For example, for the dual numbers $A = \mathbb C[x] / \langle x^2 \rangle$, the bracket relations in $\mathfrak g_A$ give a natural isomorphism $ \mathfrak g_A \cong \mathfrak g \ltimes \mathfrak g^{ab} $, where by $\mathfrak g^{ab}$ I mean the abelian Lie algebra with underlying space $\mathfrak g$. And this is indeed the Lie algebra of the tangent bundle $G \ltimes \mathfrak g$ of $G$.
Dec
26
comment Enveloping algebras of map algebras as hyperalgebras of algebraic groups
Actually, your comment about the Weil restriction for $\mathbb C$-finite $A$ is part of the motivation for my question - as a particular special case I've been trying to navigate the literature on jet group schemes (so this is the case where $A = \mathbb C[t]/ \langle t^n \rangle$ for some $n > 0$ and then considering the Weil restriction). It seems implicit in some of the literature that we do indeed obtain this enveloping algebra $U$ in this case - for example, it's true when we consider the dual numbers - but I'm having a hard time navigating the literature.
Dec
26
comment Enveloping algebras of map algebras as hyperalgebras of algebraic groups
I'm considering $\mathfrak g_A$ purely as a Lie algebra (in general infinite-dimensional) over $\mathbb C$, in which case I would expect that the augmentation $\epsilon$ should take values in $\mathbb C$, not $A$, unless I'm confused here? The element $aX$ doesn't make sense in $U$ as far as I can tell; that is, I don't expect $U$ to be an $A$-algebra. Only $X \otimes a$ makes sense, for $X \in \mathfrak g$ and $a \in A$, in which case $\Delta(X \otimes a) = (X \otimes a) \otimes 1 + 1 \otimes (X \otimes a)$. Thanks for the EGA reference - I'll look at that.
Dec
26
comment Enveloping algebras of map algebras as hyperalgebras of algebraic groups
I'm thinking of the Hopf algebra structure where we put $\epsilon(X) = 0$ and $\Delta X = X \otimes 1 + 1 \otimes X$ for all $X \in \mathfrak g_A \subseteq U(\mathfrak g_A)$ and then extend multiplicatively. Thanks for the info on representability - is there a good reference for reading about representability/non-representability of functors like this?
Dec
25
asked Enveloping algebras of map algebras as hyperalgebras of algebraic groups
Nov
28
comment Equivalence of Versions of the Affine Grassmannian
There's a discussion of the Laurent polynomial point of view in chapter 13.2 of Kumar's Kac-Moody Groups book (he gives a relation between $G(\mathbb C[t,t^{-1}])$ and the group denoted by $\mathcal G^{min}$). He relates the homogeneous spaces associated to $G(\mathbb C[t,t^{-1}])$ and $G(\mathbb C((t)))$ there, although I don't know much about the details.
Nov
19
comment Chevalley groups over $k[t]/t^n$
Okay, great - I'll look that up. Is there a standard reference for jet schemes of groups?
Nov
17
comment Chevalley groups over $k[t]/t^n$
That makes sense - I was confusing $G(A)$ with $G_A$. The viewpoint that gives $G \ltimes \mathfrak g$ when $n=2$ is exactly what I'm looking for. I'm unfamiliar with Weil restriction so I will take a look at that. Thanks for your patience in answering my questions!
Nov
17
comment Chevalley groups over $k[t]/t^n$
Ah, I think I see. I'm confusing group schemes over $\mathbb Z$ with group schemes over $k$.
Nov
17
comment Chevalley groups over $k[t]/t^n$
Let's say that $G$ is simply-connected. Then we can embed $G$ in $SL_N(k)$ for some $N$. Now consider the $k$-variety $GL_N(R_n)$ of matrices with coefficients in $R_n$. Reduction of scalars gives a map $k[GL_N(k)] \to k[GL_n(R_n)]$ so we can consider the subvariety of $GL_N(R_n)$ defined by the pushforward of $I$ in $k[GL_N(R_n)]$; this is what I thought $G(R_n)$ was, although perhaps this is naive. Somewhere I must be confused, though, because this means in particular that if $G = SL_N(k)$ then $G(R_n)$ is not the same as $SL_N(R_n)$.
Nov
16
awarded  Necromancer
Nov
16
comment Chevalley groups over $k[t]/t^n$
In this case, I suppose by "Chevalley group" I would be happy to consider the case where $G$ is a semisimple group over $k$ (and I don't care too much about isogeny, so we can take $G$ to be adjoint or simply-connected if that would make things easier).
Nov
16
revised Chevalley groups over $k[t]/t^n$
added 136 characters in body
Nov
16
asked Chevalley groups over $k[t]/t^n$
Nov
15
awarded  Nice Answer
Nov
5
awarded  Yearling