El Bachraoui's double binomial sum supercongruences

Let pp be an odd prime. Define

S(p)=k=0p1(18)kj=0k(2jj)(2k2jkj)(6j+1)(6k6j+1)S_-(p)=\sum_{k=0}^{p-1}\left(-\frac{1}{8}\right)^k\sum_{j=0}^k{2j\choose j}{2k-2j\choose k-j}(6j+1)(6k-6j+1)

and

S+(p)=k=0p1(14)kj=0k(2jj)(2k2jkj)(6j+1)(6k6j+1).S_+(p)=\sum_{k=0}^{p-1}\left(\frac{1}{4}\right)^k\sum_{j=0}^k{2j\choose j}{2k-2j\choose k-j}(6j+1)(6k-6j+1).

El Bachraoui's conjecture. For every odd prime pp,

S(p)p2(modp2),S+(p)p(modp2).S_-(p)\equiv -\frac{p}{2}\pmod{p^2},\qquad S_+(p)\equiv p\pmod{p^2}.

These strengthen the corresponding congruences modulo pp obtained from earlier qq-congruences, and are presented as two extensions conjectured by El Bachraoui. The supplied text gives no resolution status.

Sources & referencesView supporting material

Primary source

Long Li and Ji-Cai Liu, “On two supercongruences of double binomial sums”, arXiv:1912.01445 (2020).

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