Explicit basis conjecture for parabolic invariants of truncated polynomial rings

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Let Qm(n)=Fq[x1,…,xn]/(x1qm,…,xnqm){\mathcal{Q}}_m(n)=\mathbb{F}_q[x_1,\ldots,x_n]/(x_1^{q^m},\ldots,x_n^{q^m}), let P(α)P(\alpha) be the parabolic subgroup associated to a composition α=(α1,…,αℓ)\alpha=(\alpha_1,\ldots,\alpha_\ell), and let δa;b\delta_{a;b}, Φ\Phi, and Δsm\Delta_s^m be the operators and Dickson-algebra subspaces defined in the source. For a weak composition β≤α\beta\leq\alpha with ∣β∣≤m|\beta|\leq m, set Bi=β1+⋯+βiB_i=\beta_1+\cdots+\beta_i and B0=0B_0=0, and choose

fi∈ΦBi−1Δβim−Bi−1⊆ΔBim−Bi.f_i\in\Phi^{B_{i-1}}\Delta_{\beta_i}^{m-B_{i-1}}\subseteq\Delta_{B_i}^{m-B_i}.

Explicit parabolic basis conjecture. The resulting elements

δB1+1;mα1−β1(f1δB2+1;mα2−β2(⋯fℓ−1δBℓ+1;mαℓ−βℓ(fℓ)))\delta_{B_1+1;m}^{\alpha_1-\beta_1}\bigg(f_1\delta_{B_2+1;m}^{\alpha_2-\beta_2}\big(\cdots f_{\ell-1}\delta_{B_\ell+1;m}^{\alpha_\ell-\beta_\ell}(f_\ell)\big)\bigg)

form the set Bm(α)\mathcal{B}_m(\alpha), and Bm(α)\mathcal{B}_m(\alpha) is an Fq\mathbb{F}_q-basis of Qm(n)P(α){\mathcal{Q}}_m(n)^{P(\alpha)}. This refines the parabolic Hilbert-series conjecture by proposing explicit basis elements, but the source supplies no resolution status.

References

Primary source

Le Minh Ha, Nguyen Dang Ho Hai and Nguyen Van Nghia, “On Modular Invariants of Truncated Polynomial Rings in low ranks”, arXiv:2408.16250 (2025).

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