English

Determining $x$ or $y$ mod $p^2$ with $p=x^2+dy^2$

Number Theory 2015-06-09 v4 Combinatorics

Abstract

Let pp be an odd prime and let d{2,3,7}d\in\{2,3,7\}. When (dp)=1(\frac{-d}p)=1 we can write p=x2+dy2p=x^2+dy^2 with x,yZx,y\in\mathbb Z; in this paper we aim at determining xx or yy modulo p2p^2. For example, when p=x2+3y2p=x^2+3y^2, we show that if px1(mod4)p\equiv x\equiv 1\pmod 4 then k=0(p1)/2(3[3k]1)(2k+1)(2kk)2(16)k(2p)2x(modp2)\sum_{k=0}^{(p-1)/2}(3[3\mid k]-1)(2k+1)\frac{\binom{2k}k^2}{(-16)^k}\equiv\left(\frac2p\right)2x\pmod{p^2} where [3k][3\mid k] takes 11 or 00 according as 3k3\mid k or not, and that if py1(mod4)-p\equiv y\equiv 1\pmod4 then k=0(p1)/2(k3)k(2kk)2(16)k(1)(p+1)/4yk=0(p1)/2(13[3k])k(2kk)2(16)k(modp2).\sum_{k=0}^{(p-1)/2}\left(\frac k3\right)\frac{k\binom{2k}k^2}{(-16)^k} \equiv(-1)^{(p+1)/4}y\equiv\sum_{k=0}^{(p-1)/2}(1-3[3\mid k])\frac{k\binom{2k}k^2}{(-16)^k}\pmod{p^2}. We also determine k=0p1k(2kk)3mkkj<2k1j\mboxmod p\sum_{k=0}^{p-1}\frac{k\binom{2k}k^3}{m^k}\sum_{k\le j<2k}\frac1j\quad \mbox{mod}\ p for m=1,8,16,64,256,512,4096m=1,-8,16,-64,256,-512,4096.

Keywords

Cite

@article{arxiv.1210.5237,
  title  = {Determining $x$ or $y$ mod $p^2$ with $p=x^2+dy^2$},
  author = {Zhi-Wei Sun},
  journal= {arXiv preprint arXiv:1210.5237},
  year   = {2015}
}

Comments

21 pages. Mainly update references

R2 v1 2026-06-21T22:24:22.252Z