Polynomials and Representations XXXIII

We are back to the convention G = GL_n\mathbb{C} and K = U_n. We wish to focus on irreducible polynomial representations of G.

The weak Peter-Weyl theorem gives:

\displaystyle\mathcal{O}(K) \cong \bigoplus_{K-\text{irrep } V} \text{End}(V)^\vee = \bigoplus_{\text{rat. } G-\text{irrep } V} \text{End}(V)^\vee.

Theorem. Restricting the RHS to only polynomial irreducible V gives us \mathbb{C}[z_{ij}]_{1\le i, j\le n} on the LHS, where each polynomial f in z_{ij} restricts to a function K \to \mathbb{C}.


Since z_{ij} \in \mathcal{O}(K) is a matrix coefficient, we have \mathbb{C}[z_{ij}] \subseteq \mathcal{O}(K) and this is clearly a G\times G-submodule. Furthermore, as functions K\to\mathbb{C}, the z_{ij} are algebraically independent over \mathbb{C} by the main lemma here.

Since each \text{End}(V)^\vee is irreducible as a G\times G-module, \mathbb{C}[z_{ij}] corresponds to a direct sum \oplus_S \text{End}(V)^\vee over some set S of G-irreps V. It remains to show that S is precisely the set of polynomial irreps.

Next, as G\times G-representations, we have a decomposition

\displaystyle\mathbb{C}[z_{ij}] = \bigoplus_{d\ge 0} \mathbb{C}[z_{ij}]^{(d)}

into homogeneous components; each is a finite-dimensional representations of G\times G. By considering the action of 1\times G, each component is a polynomial representation of G. Hence every irrep in S must be polynomial.

Conversely, if W is any polynomial irrep of G of dimension m, upon taking a basis the action of every g\in G can be written as an m\times m matrix with entries in \mathbb{C}[z_{ij}]. Hence \mathbb{C}[z_{ij}]^m contains W; since W is irreducible, \mathbb{C}[z_{ij}] contains W. ♦


Decomposing By Degrees

Thus we have as isomorphism of G\times G-reps (*):

\displaystyle\bigoplus_{\text{poly irrep } V \text{of }G} \text{End}(V)^\vee \cong \mathbb{C}[z_{ij}] = \bigoplus_{d\ge 0} \mathbb{C}[z_{ij}]^{(d)}.

Definition. The degree of a polynomial irrep V is the unique d\ge 0 for which \mathbb{C}[z_{ij}]^{(d)} contains it.

Let us pick one particular component \mathbb{C}[z_{ij}]^{(d)}. We get a G-rep by taking the action of 1\times G. Hence its Laurent polynomial can be computed by considering the action of 1\times S on it. Since the space is spanned by monomials of degree d, we have:

Property 1. For a polynomial G-irrep V, we have \deg V = \deg \psi_V.

Recall that if G acts on V, then G\times G acts on \text{End}(V) via:

(x,y) \in G\times G, f\in \text{End}(V) \ \mapsto \ (x,y)f = \rho_V(x)\circ f\circ \rho_V(y^{-1}).

Hence, taking the dual gives:

Property 2. The action of S\times S \subset G\times G on \text{End}(V)^\vee gives the character

\displaystyle \psi_V(x_1^{-1}, \ldots, x_n^{-1}) \psi_V(y_1, \ldots, y_n)

The pair of diagonal matrices (D(x_1, \ldots, x_n), D(y_1, \ldots, y_n)) \in S\times S takes z_{ij} \mapsto x_i^{-1} z_{ij} y_j. Hence, taking the basis of monomials of degree d, we have:

Property 3. The action of S\times S\subset G\times G on \mathbb{C}[z_{ij}]^{(d)} has character:

\displaystyle \sum_{\substack{m_{11}, m_{12},\ldots, m_{nn} \ge 0\\ m_{11} + m_{12} + \ldots + m_{nn} = d}} \left( \prod_{i=1}^n \prod_{j=1}^n x_i^{-m_{ij}} y_j^{m_{ij}}\right).

Finally, from lemma 3 here and property 1 above, we see that:

Property 4. For each d, the number of polynomial G-irreps of degree d is exactly the cardinality of \{\lambda \vdash d: \lambda_1 \le n\}.


Main Theorem

Now we are ready to prove:

Theorem. A polynomial representation V of G of degree d is irreducible if and only if \psi_V is a Schur polynomial in x_1, \ldots, x_n of of degree d.


For each d, let V_{d1}, \ldots, V_{de} be the polynomial irreps of degree d; let p_{dj} := \psi_{V_{dj}}, a homogeneous symmetric polynomial in x_1, \ldots, x_n of degree d. By property 2, the character of S\times S on \mathbb{C}[z_{ij}]^{(d)} is:

\displaystyle\sum_{j} p_{dj}(x_1^{-1}, \ldots, x_n^{-1}) p_{dj}(y_1, \ldots, y_n).

By property 3 and (*), summing this over all d and j gives the power series:

\displaystyle\sum_{m_{11}, \ldots, m_{nn}\ge 0} \left( \prod_{i=1}^n \prod_{j=1}^n x_i^{-m_{ij}} y_j^{m_{ij}}\right) = \prod_{i=1}^n \prod_{j=1}^n \sum_{m\ge 0} (x_i^{-1} y_j)^m = \prod_{1\le i, j\le n} \frac 1 {1 - x_i^{-1}y_j}.

Finally by property 4, for each d, the number of p_{dj} is exactly the size of \{\lambda \vdash d : \lambda_1 \le n\}. Thus by the criterion for orthonormal basis proven (much) earlier, the \{p_{dj}\}_j forms an orthonormal basis of \Lambda_n^{(d)}. Hence, each p_{dj} is, up to sign, a Schur polynomial of degree d. Since the coefficients of p_{dj} are non-negative, they are the Schur polynomials. ♦



We have a correspondence between:


which takes V\mapsto \psi_V. Under this correspondence, for partition \lambda = (\lambda_1, \ldots, \lambda_l),

\begin{aligned}\text{Sym}^{\lambda_1} \mathbb{C}^n \otimes \ldots \otimes \text{Sym}^{\lambda_l} \mathbb{C}^n &\leftrightarrow h_\lambda(x_1, x_2, \ldots, x_n), \\\text{Alt}^{\lambda_1} \mathbb{C}^n \otimes \ldots \otimes \text{Alt}^{\lambda_l} \mathbb{C}^n &\leftrightarrow e_\lambda(x_1, x_2, \ldots, x_n), \\ \text{poly. irrep } &\leftrightarrow s_\lambda(x_1, x_2, \ldots, x_n), \\ \otimes \text{ of reps } &\leftrightarrow \text{ multiplication of polynomials},\\ \text{Hom}_G(V, W) &\leftrightarrow \text{ Hall inner product}. \end{aligned}

Indeed, the first two correspondences are obvious; the third is what we just proved. The fourth is immediate from the definition of \psi_V. The final correspondence follows from the third one. Denote V(\lambda) for the corresponding irrep of GL_n\mathbb{C}; we can now port over everything we know about symmetric polynomials, such as:

\displaystyle \begin{aligned} h_\lambda = \sum_{\mu\vdash d} K_{\mu\lambda} s_\mu \ &\implies\ \bigotimes_i \text{Sym}^{\lambda_i} \mathbb{C}^n = \bigoplus_{\mu\vdash d} V(\mu)^{\oplus K_{\mu\lambda}},\\ e_\lambda = \sum_{\mu\vdash d} K_{\overline\mu\lambda} s_\mu\ &\implies\ \bigotimes_i \text{Alt}^{\lambda_i} \mathbb{C}^n = \bigoplus_{\mu\vdash d} V(\mu)^{\oplus K_{\overline\mu\lambda}},\\ s_\lambda s_\mu = \sum_{\nu\vdash d} c_{\lambda\mu}^\nu s_\nu \ &\implies\ V(\lambda) \otimes V(\mu) \cong \bigoplus_{\nu\vdash d} V(\nu)^{\oplus c_{\lambda\mu}^\nu}.\end{aligned}

Setting \lambda = (1,\ldots, 1) for h_\lambda gives:

\displaystyle(\mathbb{C}^n)^{\otimes d} \cong \bigoplus_{\mu\vdash d} V(\mu) ^{d_\mu}

where d_\mu is the number of SYT of shape \mu.

Unfortunately, the involution map \omega : \Lambda_n \to \Lambda_n does not have a nice interpretation in our context. (No it does not take the polynomial irrep to its dual!)


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