Geodesic
The tangent function and power residues modulo primes
Czechoslovak Mathematical Journal, Tome 73 (2023) no. 3, pp. 971-978

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Let $p$ be an odd prime, and let $a$ be an integer not divisible by $p$. When $m$ is a positive integer with $p\equiv 1\pmod {2m}$ and $2$ is an $m$th power residue modulo $p$, we determine the value of the product $\prod _{k\in R_m(p)}(1+\tan (\pi ak/p))$, where $$ R_m(p)=\{0\colon k\in \mathbb Z\ \text {is an}\ m\text {th power residue modulo}\ p\}. $$ In particular, if $p=x^2+64y^2$ with $x,y\in \mathbb Z$, then $$ \prod _{k\in R_4(p)} \Big (1+\tan \pi \frac {ak}p\Big )=(-1)^{y}(-2)^{(p-1)/8}. $$
DOI : 10.21136/CMJ.2023.0395-22
Classification : 05A19, 11A15, 33B10
Keywords: power residues modulo prime; the tangent function; identity 
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Sun, Zhi-Wei. The tangent function and power residues modulo primes. Czechoslovak Mathematical Journal, Tome 73 (2023) no. 3, pp. 971-978. doi: 10.21136/CMJ.2023.0395-22

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