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Archive for April, 2010

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Let G be a group and let

\displaystyle V = \bigoplus_{n \ge 0} V_n

be a graded representation of G, i.e. a functor from G to the category of graded vector spaces with each piece finite-dimensional. Thus G acts on each graded piece V_i individually, each of which is an ordinary finite-dimensional representation. We want to define a character associated to a graded representation, but if a character is to have any hope of uniquely describing a representation it must contain information about the character on every finite-dimensional piece simultaneously. The natural definition here is the graded trace

\displaystyle \chi_V(g) = \sum_{n \ge 0} \chi_{V_n}(g) t^n.

In particular, the graded trace of the identity is the graded dimension or Hilbert series of V.

Classically a case of particular interest is when V_n = \text{Sym}^n(W^{*}) for some fixed representation W, since V = \text{Sym}(W^{*}) is the symmetric algebra (in particular, commutative ring) of polynomial functions on W invariant under G. In the nicest cases (for example when G is finite), V is finitely generated, hence Noetherian, and \text{Spec } V is a variety which describes the quotient W/G.

In a previous post we discussed instead the case where V_n = (W^{*})^{\otimes n} for some fixed representation W, hence V is the tensor algebra of functions on W. I thought it might be interesting to discuss some generalities about these graded representations, so that’s what we’ll be doing today.

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Nationals 2010

MIT is hosting the United States Rubik’s Cube Championships this summer, August 6-8. All ages welcome! Normally I wouldn’t post about such things, but

  • I happen to be a member of the Rubik’s Cube club here, and
  • Some people use the Rubik’s cube group to motivate group theory. I’m a fan of hands-on mathematics, and there’s a lot to learn from the cube; for example, you quickly understand that groups are not in general commutative. The Rubik’s cube itself is also a good example of a torsor.

Actually, just so this post has some mathematical content, there’s something about the Rubik’s cube group that is probably very simple to explain, but which I don’t completely understand. It’s a common feature of Rubik’s cube algorithms that they need to switch around some parts of the cube without disturbing others; in other words, the corresponding permutation needs to have a lot of fixed points. This seems to be done by writing down a lot of commutators, but I’m not familiar with any statements in group theory of the form “commutators tend to have fixed points.” Can anyone explain this?

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