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 A087012 #24 Sep 25 2024 19:22:37
%S A087012 1,3,4,5,8,10,11,12,13,15,20,22,23,24,25,26,31,34,35,37,49,50,52,53,
%T A087012 57,58,59,62,63,69,72,73,75,79,82,83,84,85,86,91,92,93,94,95,97,99,
%U A087012 141,147,148,149,152,153,164,165,168,175,176,182,183,187,188,189,200,244,245
%N A087012 Numbers m such that the number of primes of form 4*k+1 between m and 2*m equals the number of primes of form 4*k+3 between m and 2*m (inclusive).
%H A087012 Amiram Eldar, <a href="/A087012/b087012.txt">Table of n, a(n) for n = 1..10000</a>
%t A087012 seqQ[n_] := Module[{c1 = 0, c3 = 0}, Do[If[Mod[k, 4] == 1 && PrimeQ[k], c1++]; If[Mod[k, 4] == 3 && PrimeQ[k], c3++], {k, n, 2 n}]; c1 == c3]; Select[Range[250], seqQ] (* _Amiram Eldar_, Dec 16 2019 *)
%t A087012 npfQ[n_]:=With[{prs=Select[Range[n,2n],PrimeQ]},Length[Select[prs,Mod[#,4]==1&]]==Length[Select[prs,Mod[#,4]==3&]]]; Select[ Range[ 250],npfQ] (* _Harvey P. Dale_, Sep 25 2024 *)
%o A087012 (PARI) for(m=1,250,my(k1=0,k3=0);forprime(p=m,2*m,if(p%4==1,k1++);if(p%4==3,k3++));if(k1==k3,print1(m," "))) \\ _Hugo Pfoertner_, Dec 16 2019
%o A087012 (Magma) f:=func<n,r|#[p:p in PrimesInInterval(n, 2*n)| p mod 4 eq r]>; [k:k in [1..250]|f(k,1) eq f(k,3)]; // _Marius A. Burtea_, Dec 16 2019
%Y A087012 Cf. A035250, A087010, A087011.
%K A087012 nonn
%O A087012 1,2
%A A087012 _Jason Earls_, Jul 30 2003