# A parametric congruence arising from Orr's identity

Research paper by **Chen Wang, Zhi-Wei Sun**

Indexed on: **05 Mar '21**Published on: **04 Mar '21**Published in: **arXiv - Mathematics - Number Theory**

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#### Abstract

For any $m,n\in\mathbb{N}=\{0,1,2\ldots\}$, the truncated hypergeometric
series ${}_{m+1}F_m$ is defined by $$
{}_{m+1}F_m\bigg[\begin{matrix}x_0&x_1&\ldots&x_m\\
&y_1&\ldots&y_m\end{matrix}\bigg|z\bigg]_n=\sum_{k=0}^n\frac{(x_0)_k(x_1)_k\cdots(x_m)_k}{(y_1)_k\cdots(y_m)_k}\cdot\frac{z^k}{k!},
$$ where $(x)_k=x(x+1)\cdots(x+k-1)$ is the Pochhammer symbol. Let $p$ be an
odd prime. Then for $\alpha,z\in\mathbb{Z}_p$ with $\langle
-\alpha\rangle_p\equiv0\pmod{2}$ we mainly prove the following congruence
arising from Orr's identity: $$
{}_2F_1\bigg[\begin{matrix}\frac12\alpha&\frac32-\frac12\alpha\\
&1\end{matrix}\bigg|z\bigg]_{p-1}{}_2F_1\bigg[\begin{matrix}\frac12\alpha&\frac12-\frac12\alpha\\
&1\end{matrix}\bigg|z\bigg]_{p-1}\equiv{}_3F_2\bigg[\begin{matrix}\alpha&2-\alpha&\frac12\\
&1&1\end{matrix}\bigg|z\bigg]_{p-1}\pmod{p^2}, $$ where $\langle x\rangle_p$
denotes the least nonnegative residue of $x$ modulo $p$ for any
$x\in\mathbb{Z}_p$. As a corollary, we deduce that $$
\sum_{k=0}^{p-1}(b^2k+b-1)\frac{\binom{2k}{k}}{4^k}\binom{-1/b}{k}\binom{1/b-1}{k}\equiv0\pmod{p^2},
$$ where $b\in\mathbb{Z}^{+}$ and $p$ is a prime with $p\equiv\pm1\pmod{b}$ and
$\langle -1/b\rangle_p\equiv0\pmod{2}$. This partially confirms a conjectural
congruence of the second author [Nanjing Univ. J. Math. Biquarterly 36 (2019),
no. 1, 1--99].