# Semi-invariant of a quiver

In mathematics, given a quiver Q with set of vertices Q$_0$ and set of arrows Q$_1$, a representation of Q assigns a vector space V$_i$ to each vertex and a linear map V(α): V(s(α)) → V(t(α)) to each arrow α, where s(α), t(α) are the starting and the ending vertices of α. Given an element d ∈ $\mathbb {N}$$^{Q0}$, the set of representations of Q with dim V$_i$ = d(i) for each i has a vector space structure.
It is naturally endowed with an action of the algebraic group Π$_{i∈Q0}$ GL(d(i)) by simultaneous base change. Such action induces one on the ring of functions. The ones which are invariants up to a character of the group are called semi-invariants. They form a ring whose structure reflects representation-theoretical properties of the quiver.

## Definitions

Let Q = (Q$_0$,Q$_1$,s,t) be a quiver. Consider a dimension vector d, that is an element in $\mathbb {N}$$^{Q0}$. The set of d-dimensional representations is given by
    $\operatorname {Rep} (Q,\mathbf {d} ):=\{V\in \operatorname {Rep} (Q):V_{i}=\mathbf {d} (i)\}$
Once fixed bases for each vector space V$_i$ this can be identified with the vector space
    $\bigoplus _{\alpha \in Q_{1}}\operatorname {Hom} _{k}(k^{\mathbf {d} (s(\alpha ))},k^{\mathbf {d} (t(\alpha ))})$
Such affine variety is endowed with an action of the algebraic group GL(d) := Π$_{i∈Q0}$ GL(d(i)) by simultaneous base change on each vertex:
    ${\begin{array}{ccc}GL(\mathbf {d} )\times \operatorname {Rep} (Q,\mathbf {d} )&\longrightarrow &\operatorname {Rep} (Q,\mathbf {d} )\\{\Big(g_{i}),(V_{i},V(\alpha )){\Big )}&\longmapsto &(V_{i},g_{t(\alpha )}\cdot V(\alpha )\cdot g_{s(\alpha )}^{-1})\end{array}}$
By definition two modules M,N ∈ Rep(Q,d) are isomorphic if and only if their GL(d)-orbits coincide.
We have an induced action on the coordinate ring k[Rep(Q,d)] by defining:
    ${\begin{array}{ccc}GL(\mathbf {d} )\times k[\operatorname {Rep} (Q,\mathbf {d} )]&\longrightarrow &k[\operatorname {Rep} (Q,\mathbf {d} )]\\(g,f)&\longmapsto &g\cdot f(-):=f(g^{-1}.-)\end{array}}$

### Polynomial invariants

An element f ∈ k[Rep(Q,d)] is called an invariant (with respect to GL(d)) if g⋅f = f for any g ∈ GL(d). The set of invariants
    $I(Q,\mathbf {d} ):=k[\operatorname {Rep} (Q,\mathbf {d} )]^{GL(\mathbf {d} )}$
is in general a subalgebra of k[Rep(Q,d)].

#### Example

Consider the 1-loop quiver Q:
For d = (n) the representation space is End(k$^n$) and the action of GL(n) is given by usual conjugation. The invariant ring is
    $I(Q,\mathbf {d} )=k[c_{1},\ldots ,c_{n}]$
where the c$_i$s are defined, for any A ∈ End(k$^n$), as the coefficients of the characteristic polynomial
    $\det(A-t\mathbb {I} )=t^{n}-c_{1}(A)t^{n-1}+\cdots +(-1)^{n}c_{n}(A)$

### Semi-invariants

In case Q has neither loops nor cycles, the variety k[Rep(Q,d)] has a unique closed orbit corresponding to the unique d-dimensional semi-simple representation, therefore any invariant function is constant.
Elements which are invariants with respect to the subgroup SL(d) := Π$_{i∈Q0}$ SL(d(i)) form a ring, SI(Q,d), with a richer structure called ring of semi-invariants. It decomposes as
    $SI(Q,\mathbf {d} )=\bigoplus _{\sigma \in \mathbb {Z} ^{Q_{0}}}SI(Q,\mathbf {d} )_{\sigma }$
where
    $SI(Q,\mathbf {d} )_{\sigma }:=\{f\in k[\operatorname {Rep} (Q,\mathbf {d} )]:g\cdot f=\prod _{i\in Q_{0}}\det(g_{i})^{\sigma _{i}}f,\forall g\in GL(\mathbf {d} )\}.$
A function belonging to SI(Q,d)$_σ$ is called semi-invariant of weight σ.

#### Example

Consider the quiver Q:
    $1{\xrightarrow {\ \ \alpha \ }}2$
Fix d = (n,n). In this case k[Rep(Q,(n,n))] is congruent to the set of n-by-n matrices: M(n). The function defined, for any B ∈ M(n), as det$^u$(B(α)) is a semi-invariant of weight (u,−u) in fact
    $(g_{1},g_{2})\cdot {\det }^{u}(B)={\det }^{u}(g_{2}^{-1}Bg_{1})={\det }^{u}(g_{1}){\det }^{-u}(g_{2}){\det }^{u}(B)$
The ring of semi-invariants equals the polynomial ring generated by det, i.e.
    ${\mathsf {SI}}(Q,\mathbf {d} )=k[\det ]$

## Characterization of representation type through semi-invariant theory

For quivers of finite representation-type, that is to say Dynkin quivers, the vector space k[Rep(Q,d)] admits an open dense orbit. In other words, it is a prehomogenous vector space. Sato and Kimura described the ring of semi-invariants in such case.

### Sato–Kimura theorem

Let Q be a Dynkin quiver, d a dimension vector. Let Σ be the set of weights σ such that there exists f$_σ$ ∈ SI(Q,d)$_σ$ non-zero and irreducible. Then the following properties hold true.
i) For every weight σ we have dim$_k$ SI(Q,d)$_σ$ ≤ 1.
ii) All weights in Σ are linearly independent over $\mathbb {Q}$.
iii) SI(Q,d) is the polynomial ring generated by the f$_σ$'s, σ ∈ Σ.
Furthermore, we have an interpretation for the generators of this polynomial algebra. Let O be the open orbit, then k[Rep(Q,d)] \ O = Z$_1$ ∪ ... ∪ Z$_t$ where each Z$_i$ is closed and irreducible. We can assume that the Z$_i$s are arranged in increasing order with respect to the codimension so that the first l have codimension one and Z$_i$ is the zero-set of the irreducible polynomial f$_1$, then SI(Q,d) = k[f$_1$, ..., f$_l$].

#### Example

In the example above the action of GL(n,n) has an open orbit on M(n) consisting of invertible matrices. Then we immediately recover SI(Q,(n,n)) = k[det].
Skowronski–Weyman provided a geometric characterization of the class of tame quivers (i.e. Dynkin and Euclidean quivers) in terms of semi-invariants.

### Skowronski–Weyman theorem

Let Q be a finite connected quiver. The following are equivalent:
i) Q is either a Dynkin quiver or a Euclidean quiver.
ii) For each dimension vector d, the algebra SI(Q,d) is complete intersection.
iii) For each dimension vector d, the algebra SI(Q,d) is either a polynomial algebra or a hypersurface.

#### Example

Consider the Euclidean quiver Q:
Pick the dimension vector d = (1,1,1,1,2). An element V ∈ k[Rep(Q,d)] can be identified with a quadruple (A$_1$, A$_2$, A$_3$, A$_4$) of matrices in M(1,2). Call D$_{i,j}$ the function defined on each V as det(A$_i$,A$_j$). Such functions generate the ring of semi-invariants:
    $SI(Q,\mathbf {d} )={\frac {k[D_{1,2},D_{3,4},D_{1,4},D_{2,3},D_{1,3},D_{2,4}]}{D_{1,2}D_{3,4}+D_{1,4}D_{2,3}-D_{1,3}D_{2,4}}}$