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Questions about the branch of algebra that deals with groups.

1 vote
0 answers
216 views

Are there algorithms and/or criteria for determining if a group has a presentation with all ...

Suppose $G = F/R$ is a finitely presented group with $F = F_n$ a free group and $R$ the normal closure of words $W_1, \dots, W_p$, not all of which are products of commutators. An obvious necessary co …
Nick Salter's user avatar
  • 2,830
24 votes
3 answers
3k views

Combinatorial Techniques for Counting Conjugacy Classes

The number of conjugacy classes in $S_n$ is given by the number of partitions of $n$. Do other families of finite groups have a highly combinatorial structure to their number of conjugacy classes? For …
Nick Salter's user avatar
  • 2,830
6 votes
Accepted

Lifting from mapping class group to groups of homeo/diffeomorphisms

I'm not sure if this is exactly what you were asking about, but one way of parsing your question is to ask when a homomorphism from $G$ to $\operatorname{MCG}(M)$ is induced from a homomorphism $G \to …
Nick Salter's user avatar
  • 2,830
1 vote

Is the *unreduced* Burau representation unitary?

After lots of trial and error, it looks like I can give an explicit construction of such a Hermitian form, although it's utterly mysterious to me where such a thing comes from! Let $e_1, \dots, e_n$ …
Nick Salter's user avatar
  • 2,830
7 votes
2 answers
484 views

Is the *unreduced* Burau representation unitary?

In a 1984 paper, Squier wrote down an explicit Hermitian form $J$ relative to which the reduced Burau representation is unitary. The form is valued in the ring $\mathbb{Z}[s^{\pm 1}]$, where $s^2 = t$ …
Nick Salter's user avatar
  • 2,830
11 votes
1 answer
777 views

What is an interpretation of the relation in the cohomology of the pure braid groups?

In 1968, Arnol'd proved that the integral cohomology of the pure braid group $P_n$ is isomorphic to the exterior algebra generated by the collection of degree-one classes $\omega_{i,j}\ (1 \le i < j \ …
Nick Salter's user avatar
  • 2,830