# The symmetric functions catalog

An overview of symmetric functions and related topics

2020-08-31

## Character symmetric functions

In [OZ18], the authors define the irreducible character basis, $\{\chSym_\lambda\},$ the induced character basis, $\{\indCharBasis_\lambda\},$ and the induced trivial character basis, $\{\indTrivBasis_\lambda\}.$ The induced character basis was previously studied by D. Speyer and S. Assaf [AS19d] under the name stable Specht polynomials.

In order to define these three families, we need some additional notation. First, let

$\bar{\powerSum}_{i^r} \coloneqq i^r \left( \frac{1}{i} \sum_{d \mid i } \mu(i/d) \powerSum_d \right)_i \text{ and } \hat{\powerSum}_{i^r} \coloneqq \sum_{k=0}^r (-1)^{r-k} \binom{r}{k} \bar{\powerSum}_{i^k},$

where we use the notation of falling factorial in the first expression. With these definitions, we can define $\bar{\powerSum}_\gamma$ and $\hat{\powerSum}_\gamma,$ for partitions $\gamma.$ Finally, we set

$\chSym_\lambda \coloneqq \sum_{\gamma} \chi^{\lambda}(\gamma) \frac{ \hat{\powerSum}_\gamma }{z_\gamma} \qquad \indCharBasis_\lambda \coloneqq \sum_{\gamma} \chi^{\lambda}(\gamma) \frac{ \bar{\powerSum}_\gamma }{z_\gamma} \quad \indTrivBasis_\lambda \coloneqq \sum_{\gamma} \langle \completeH_\gamma, \powerSum_\gamma \rangle \frac{ \bar{\powerSum}_\gamma }{z_\gamma} \quad$

The irreducible character symmetric functions $\{\chSym_\lambda\}$ were introduced by Orellana and Zabrocki already in [OZ16]. They are non-homogeneous symmetric functions, and may alternatively be defined as follow.

Let $\lambda$ be a fixed partition, and $n \geq |\lambda|+\lambda_1.$ Then for all partitions $\gamma \vdash n,$

$\chSym_\lambda(\zeta_{1},\dotsc,\zeta_{n}) = \chi^{(n-|\lambda|,\lambda)}(\gamma)$

where $\zeta_{1},\dotsc,\zeta_{n}$ are the eigenvalues of a permutation matrix with cycle structure $\gamma.$ This property uniquely defines the $\chSym_\lambda.$

This definition makes the connection with character polynomials evident, see the paper [GG09] for more background.

### Properties

The multiplicative structure constants are given by the reduced Kronecker coefficients,

$\chSym_\lambda \chSym_\mu = \sum_{\nu} \bar{g}^{\nu}_{\lambda,\mu} \chSym_\nu.$

Moreover, this property plus $\schurS_{1^r} = \chSym_{1^r}+\chSym_{1^{r-1}}$ uniquely defines the character symmetric functions.

The $\{\chSym_\mu\}$ are the unique set of solutions to the system of equations

$\schurS_\lambda = \sum_{\mu : |\mu| \leq |\lambda|} A_{\lambda,(n-|\mu|,\mu)} \chSym_\mu$

for all $n$ sufficiently large. Here, $A_{\lambda,\mu} = \langle \schurS_\lambda, \schurS_\mu[1 + \completeH_1 + \completeH_2 + \dotsb \rangle,$ where we use plethystic notation. It is an open problem to combinatorially describe the $A_{\lambda,\mu}.$

For a brief overview, see this OPAC blog post.

Orellana and Zabrocki prove a Murnaghan–Nakayama type identity.