Plethysm and orbit harmonics

Abstract: Let $Π{(b^a)}$ be the locus of unordered set partitions of $[ab]$ with $a$ blocks of size $b$. We embed unordered set partitions of $[n]$ into the affine space $\mathbb{C}^{\binom{[n]}{2}}$ with coordinate ring $\mathbb{C}\Big[\mathbf{x}{\binom{[n]}{2}}\Big]$. Then, we apply orbit harmonics to $Π{(2^a)}$ and $Π{(a^2)}$, yielding graded $\mathfrak{S}{2a}$-modules whose graded character formulae respectively refine the Schur expansions of $h_a[h_2]$ and $h_2[h_a]$ according to $λ_1$. We further extend this $λ_1$-separation phenomenon to quotients of $\mathbb{C}^{\binom{[n]}{2}}$ where $n$ is odd. Combining $Π{(b^a)},Π_{(a^b)}$ and orbit harmonics, we propose a conjecture related to Foulkes' conjecture, and we prove the special case $b=2$. We also apply orbit harmonics to the locus $Π{n,m}$ of unordered set partitions of $[n]$ without blocks of size greater than $m$, yielding a graded $\mathfrak{S}_n$-module $R(Π{n,m})$. We determine the standard monomial basis of $R(Π_{n,m})$ with respect to any monomial order, as well as its graded character formula.