Cheong and Kaplan's polynomial push-forward cokernel conjecture

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Let P(t)∈Zp[t]P(t) \in \mathbb{Z}_p[t] be a non-constant monic square-free polynomial, written as

P(t)=P1(t)⋯Pl(t),P(t)=P_1(t)\cdots P_l(t),

where each Pj(t)∈Zp[t]P_j(t)\in\mathbb{Z}_p[t] is monic and its reduction Pˉj(t)\bar{P}_j(t) modulo pp is distinct and irreducible in Fp[t]\mathbb{F}_p[t]. For a finite Zp[t]/(P(t))\mathbb{Z}_p[t]/(P(t))-module GG, let An∈Mn(Fp)A_n\in\mathrm{M}_n(\mathbb{F}_p) satisfy cok⁡(Pˉ(An))≃Fp[t]G/pG\operatorname{cok}(\bar{P}(A_n))\simeq_{\mathbb{F}_p[t]}G/pG. Write qj=pdeg⁡(Pj)q_j=p^{\deg(P_j)}, let Fqj=Fp[t]/(Pˉj(t))\mathbb{F}_{q_j}=\mathbb{F}_p[t]/(\bar{P}_j(t)), and set rqj(G)=dim⁡Fqj(G/pG⊗Fp[t]Fqj)r_{q_j}(G)=\dim_{\mathbb{F}_{q_j}}(G/pG\otimes_{\mathbb{F}_p[t]}\mathbb{F}_{q_j}). Cheong and Kaplan's conjecture. One must have

Prob⁡X∈Mn(Zp)Haar(cok⁡(P(X))≃Zp[t]G∣X≡An(modp))=1∣Aut⁡Zp[t](G)∣∏j=1lprqj(G)2∏i=1rqj(G)(1−qj−i)2.\operatorname{Prob}_{X\in\mathrm{M}_n(\mathbb{Z}_p)^{\mathrm{Haar}}}\left(\operatorname{cok}(P(X))\simeq_{\mathbb{Z}_p[t]}G\mid X\equiv A_n\pmod p\right)=\frac{1}{|\operatorname{Aut}_{\mathbb{Z}_p[t]}(G)|}\prod_{j=1}^{l}p^{r_{q_j}(G)^2}\prod_{i=1}^{r_{q_j}(G)}(1-q_j^{-i})^2.

This conjecture generalizes results of Friedman and Washington and subsequent special cases proved by the authors, Cheong and Kaplan, and Cheong, Liang, and Strand; it predicts the conditional distribution of cokernels of polynomial images of random integral matrices when the polynomial has distinct irreducible residue factors.

References

Primary source

Gilyoung Cheong and Yifeng Huang, “The cokernel of a polynomial push-forward of a random integral matrix with concentrated residue”, arXiv:2310.09491 (2023).

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