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Linear representations of algebras and groups, Lie theory, associative algebras, multilinear algebra.

4 votes
Accepted

Finding exceptional regular representations of $\tilde{D}_4$ efficiently

The AR quiver of the regular representations of an affine quiver consists of infinitely many "tubes". A tube of rank $r$ has $r$ modules on what you call the border. Let me number them $B_1, B_2, \dot …
Hugh Thomas's user avatar
  • 6,327
17 votes

Statistics of irreps of S_n that can be read off the Young diagram, and consequences of Kero...

There is a beautiful interpretation of $f(\chi)$ (that is to say, of the length of the first column of the partition), though it isn't very representation-theoretic. One way to generate Plancherel mea …
LSpice's user avatar
  • 12.9k
12 votes
2 answers
323 views

Easy way to understand theta basis for X-cluster algebras of finite type?

For $\mathcal A$-cluster algebras of finite type, it is very easy to describe the theta-basis: it consists of the cluster monomials. Is there any similarly easy way to describe the theta-basis for $\m …
6 votes

Quiver representations of type $D_n$ mutation class

The quiver given in the question has five simple modules, six which correspond to a single arrow, and the remaining representations have support as follows: 123, 124, 125, 235, 345, 1235, 12235 (note …
Hugh Thomas's user avatar
  • 6,327
3 votes

Some interesting and elementary topics with connections to the representation theory?

One example of an elementary application of cluster algebras is the proof that the Somos-4 and Somos-5 sequences, which are defined by a simple recursion, are integral. This is so because the entries …
Hugh Thomas's user avatar
  • 6,327
5 votes

What's an illustrative example of a tame algebra?

For the Kronecker quiver (two vertices, two arrows in the same direction) and dimension vector (1,1), over an algebraically closed ground field, the indecomposables are naturally parameterized by poin …
Mike Pierce's user avatar
  • 1,161
32 votes
3 answers
4k views

Replacing triangulated categories with something better

Gelfand and Manin in their 1988 book on homological algebra write that the non-functoriality of cones means that "something is going wrong in the axioms of a triangulated category. Unfortunately at th …
10 votes
Accepted

Is it possible that the GHKK canonical basis for cluster algebras is the Lusztig/Kashiwara d...

I think there is good reason to think the answer is "no". In rank 2, the theta basis agrees with the greedy basis (arXiv:1508.01404). Greedy basis elements are indecomposable positive elements (see …
Hugh Thomas's user avatar
  • 6,327
6 votes
1 answer
200 views

Closures of orbits in the space of representations of a quiver

Let $Q$ be a quiver, and let $d=(d_i)$ be a dimension vector. We can consider Rep($Q,d$), the affine space consisting of representations of $Q$ with dimension vector $d$. The general linear $GL(d)= \p …
3 votes
Accepted

Closures of orbits in the space of representations of a quiver

It turns out the answer is "no". There is an example in section 3.4 of Riedtmann's paper "Degenerations for representations of quivers with relations", Ann. sci. Éc. Norm. Sup. v. 19 (1986), 275-301. …
Hugh Thomas's user avatar
  • 6,327
3 votes
Accepted

Intuition for the Euler form in a finitary category

This answer perhaps says things that are all obvious to the OP. $\textrm{Ext}^i(A,B)$ is a vector space over the ground field, so its cardinality is $q^d$ where $d$ is the dimension of the vector sp …
Hugh Thomas's user avatar
  • 6,327
4 votes

History of an open problem on partial tilting modules

There is relevant information here, including a statement of the conjecture (as Conjecture 5.1). http://www.math.uni-bonn.de/people/schroer/fd-problems-files/FD-RigidModulesConj.pdf That preprint …
Hugh Thomas's user avatar
  • 6,327
5 votes
0 answers
363 views

Kac's theorem for quiver representations over an arbitrary ground field

Let $Q$ be a quiver without loops (cycles of length 1). Kac proved that if $K$ is algebraically closed, the dimension vectors of indecomposable representations of $Q$ over $K$ are exactly the positiv …
23 votes
Accepted

Why are coherent sheaves on $\Bbb P^1$ derived equivalent to representations of the Kronecke...

Let $\mathcal O$ be the structure sheaf of $\mathbb P^1$. Then $\mathcal O \oplus \mathcal O(1)$ is rigid and generates the derived category of coherent sheaves on $\mathbb P^1$. Thus, it is a tilti …
Hugh Thomas's user avatar
  • 6,327
2 votes

Bijection on tilting modules

I am going to give a negative answer for the first question, under a stronger notion of canonicity. The approach I want to take is to consider the poset of tilting modules. They are ordered by in …
Hugh Thomas's user avatar
  • 6,327

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