Wei's fourth-power q-supercongruence for n congruent to 3 modulo 4

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Let nn be a positive integer with n≡3(mod4)n\equiv3\pmod4, let [n]=(1−qn)/(1−q)[n]=(1-q^n)/(1-q), let (a;q)k(a;q)_k be the qq-shifted factorial, and let (a1,…,ar;q)k=∏i=1r(ai;q)k(a_1,\ldots,a_r;q)_k=\prod_{i=1}^r(a_i;q)_k. Set M=(n−1)/2M=(n-1)/2 or M=n−1M=n-1. Wei's conjecture.

∑k=0M(−1)k[4k+1](q;q2)k4(q2;q4)k(q2;q2)k4(q4;q4)kqk≡[n]2(q3;q4)(n−1)/2(q5;q4)(n−1)/2q(1−n)/2(mod[n]Φn(q)4).\sum_{k=0}^{M}(-1)^k[4k+1]\frac{(q;q^2)_k^4(q^2;q^4)_k}{(q^2;q^2)_k^4(q^4;q^4)_k}q^k\equiv[n]^2\frac{(q^3;q^4)_{(n-1)/2}}{(q^5;q^4)_{(n-1)/2}}q^{(1-n)/2}\pmod{[n]\Phi_n(q)^4}.

In particular, for p≡3(mod4)p\equiv3\pmod4, the source proposes

∑k=0(pr−1)/2(−1)k(4k+1)(12)k5k!5≡p2r(34)(pr−1)/2(54)(pr−1)/2(modpr+4).\sum_{k=0}^{(p^r-1)/2}(-1)^k(4k+1)\frac{(\frac{1}{2})_k^5}{k!^5}\equiv p^{2r}\frac{(\frac{3}{4})_{(p^r-1)/2}}{(\frac{5}{4})_{(p^r-1)/2}}\pmod{p^{r+4}}.

This is motivated by numerical calculations and strengthens the earlier cubic-power qq-congruence; the source gives no proof or resolution evidence.

References

Primary source

Chuanan Wei, “Some q-supercongruences modulo the fourth power of a cyclotomic polynomial”, arXiv:2005.14196 (2020).

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