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Questions about the branch of algebra that deals with groups.

32 votes

What is the difference between PSL_2 and PGL_2?

As Kevin says, the "right" definition of ${\rm{PSL}}_n$ is as representing the quotient sheaf ${\rm{SL}}_n/\mu_n$, just as one defines ${\rm{PSO}}(q) = {\rm{SO}}(q)/Z_{{\rm{SO}}(q)}$ (with $Z_G$ denot …
BCnrd's user avatar
  • 7,108
19 votes

Is a quotient of a reductive group reductive?

It is important in answering this question that one can extend scalars to a perfect (e.g., algebraically closed) ground field, as was implicit in many of the other answers even if not said explicitly. …
BCnrd's user avatar
  • 7,108
10 votes

Unipotent linear algebraic groups

Let U be a smooth connected unipotent group over an arbitrary field k, and let T be a k-split k-torus equipped with a left action on U such that the T-action on Lie(U) contains no occurrence of the tr …
BCnrd's user avatar
  • 7,108
7 votes

Algebraic groups of relative rank 1

Assume $n \ge 5$ to avoid interference with exceptional isomorphisms among small Dynkin diagrams. The case of odd $n$ should be easier, since in that case (with $n \ge 5$) there are no nontrivial dia …
BCnrd's user avatar
  • 7,108