This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A102271 #33 Aug 12 2025 02:38:16
%S A102271 3,7,19,31,103,139,199,223,271,283,307,367,439,523,607,619,643,691,
%T A102271 727,787,811,859,1039,1063,1123,1231,1279,1291,1399,1447,1459,1483,
%U A102271 1531,1543,1567,1627,1699,1783,1867,1879,1951,1987,2131,2203,2239,2287,2371,2383
%N A102271 Primes of the form 3*x^2 + 7*y^2.
%C A102271 Primes p such that Q(sqrt(-21p)) has genus characters chi_{-3} = +1, chi_{-7} = -1.
%H A102271 Ray Chandler, <a href="/A102271/b102271.txt">Table of n, a(n) for n = 1..10000</a> (First 1000 terms from Vincenzo Librandi).
%H A102271 H. Cohn and J. C. Lagarias, <a href="http://doi.org/10.1090/S0025-5718-1983-0717716-8">On the existence of fields governing the 2-invariants of the classgroup of Q(sqrt{dp}) as p varies</a>, Math. Comp. 41 (1983), 711-730.
%H A102271 N. J. A. Sloane et al., <a href="https://oeis.org/wiki/Binary_Quadratic_Forms_and_OEIS">Binary Quadratic Forms and OEIS</a> (Index to related sequences, programs, references)
%F A102271 The primes are congruent to {3, 7, 19, 31, 55} (mod 84). - _T. D. Noe_, May 02 2008
%t A102271 m=3; n=7; pLst={}; lim=3000; xMax=Sqrt[lim/m]; yMax=Sqrt[lim/n]; Do[p=m*x^2+n*y^2; If[p<lim && PrimeQ[p], AppendTo[pLst, p]], {x, xMax}, {y, yMax}]; Union[pLst] (* _T. D. Noe_, May 05 2005 *)
%t A102271 QuadPrimes2[3, 0, 7, 10000] (* see A106856 *)
%o A102271 (Magma) [p: p in PrimesUpTo(3000) | p mod 84 in [3, 7, 19, 31, 55]]; // _Vincenzo Librandi_, Jul 19 2012
%o A102271 (PARI) list(lim)=my(v=List(),w,t); for(x=0, sqrtint(lim\3), w=3*x^2; for(y=0, sqrtint((lim-w)\7), if(isprime(t=w+7*y^2), listput(v,t)))); Set(v) \\ _Charles R Greathouse IV_, Feb 09 2017
%Y A102271 Cf. A102269-A102275, A139827.
%K A102271 nonn,easy
%O A102271 1,1
%A A102271 _N. J. A. Sloane_, Feb 19 2005