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 A079261 #25 Apr 26 2025 17:52:57
%S A079261 0,0,1,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,
%T A079261 0,0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,1,0,
%U A079261 0,0,1,0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0
%N A079261 Characteristic function of primes of form 4n+3 (1 if n is prime of form 4n+3, 0 otherwise).
%C A079261 Let M(n) denote the n X n matrix m(i,j)=0 if n divides ij-1, m(i,j) = 1 otherwise then det(M(n))=+1 if and only if n is prime ==3 (mod 4).
%C A079261 a(A002145(n)) = 1; a(A145395(n)) = 0. [From _Reinhard Zumkeller_, Oct 12 2008]
%C A079261 a(n) * A151763(n) = - a(n).
%H A079261 Reinhard Zumkeller, <a href="/A079261/b079261.txt">Table of n, a(n) for n = 1..10000</a>
%H A079261 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%F A079261 a(n) = - A010051(n) * A011764(n+1). [_Reinhard Zumkeller_, Oct 06 2011]
%t A079261 Table[If[PrimeQ[n]&&Mod[n,4]==3,1,0],{n,120}] (* _Harvey P. Dale_, Apr 26 2025 *)
%o A079261 (PARI) { a(n)=isprime(n)*if(n%4-3,0,1) }; vector(100,n,a(n))
%o A079261 (Haskell)
%o A079261 a079261 n = fromEnum $ n `mod` 4 == 3 && a010051 n == 1
%o A079261 -- _Reinhard Zumkeller_, Oct 06 2011
%Y A079261 Cf. A002145, A079260.
%Y A079261 Cf. A066490 (partial sums).
%K A079261 nonn
%O A079261 1,1
%A A079261 _Benoit Cloitre_, Feb 04 2003