2019-05-20

## Boolean product polynomials

The boolean product polynomials is a family of polynomials defined as follows:

\[ \booleanProduct_{k}(\xvec_n) \coloneqq \prod_{ 1 \leq i_1 \lt i_2 \lt \dotsb \lt i_k \leq n } (x_{i_1} + x_{i_2}+\dotsb + x_{i_k}). \]These were introduced by L. Billera, S. Billey, and V. Tewari, in an FPSAC abstract, and inspired by certain applications in hyperplane arrangement, among other things.

### Schur positivity

In [BRT19] it is proved that the
functions $\booleanProduct_{k}(\xvec_n)$ are Schur-positive.
The authors use a new method for proving Schur positivity, called *Chern plethysm*.

**Problem.**

Find a combinatorial interpretation for the coefficients in the expansion

\[ \booleanProduct_{k}(\xvec_n) = \sum_{\lambda} \delta^{n,k}_{\lambda} \schurS_\lambda(\xvec_n). \]Find natural $\symS_n$-modules whose Frobenius characteristic is $\booleanProduct_{n,k}(\xvec).$

### Bivariate extension

There is a bivariate extension,

\[ \booleanProduct_{k,\ell}(\xvec_n;\yvec_m) = \coloneqq \prod_{ \substack{ 1 \leq i_1 \lt i_2 \lt \dotsb \lt i_k \leq n \\ 1 \leq j_1 \lt j_2 \lt \dotsb \lt j_\ell \leq m }} (x_{i_1} + x_{i_2}+\dotsb + x_{i_k} + y_{j_1} + y_{j_2}+\dotsb + y_{j_\ell} ). \]One can show using representation theory (see [BRT19]) that the $a_{\lambda,\mu}$ in

\[ \booleanProduct_{k,\ell}(\xvec_n;\yvec_m) = \sum_{\lambda,\mu} a_{\lambda,\mu} \schurS_\lambda(\xvec_n)\schurS_\mu(\yvec_m). \]are non-negative integers. When $k=\ell = 1,$ this reduces to the dual Cauchy identity.

## References

- [BRT19] Sara C. Billey, Brendon Rhoades and Vasu Tewari. Boolean product polynomials, Schur positivity, and Chern plethysm. arXiv e-prints, 2019.