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 A319660 #18 Feb 24 2021 08:17:17
%S A319660 0,0,0,1,0,0,0,1,1,0,0,1,1,0,0,0,0,0,1,1,1,0,0,1,1,1,1,0,0,0,1,0,1,1,
%T A319660 0,0,0,1,1,0,2,0,1,0,1,1,0,0,2,1,0,1,0,2,1,0,1,0,0,1,1,1,1,1,0,0,1,1,
%U A319660 1,0,1,1,0,1,0,0,1,0,0,1,1,2,1,1,1,1,0
%N A319660 2-rank of the class group of imaginary quadratic field with discriminant -k, k = A039957(n).
%C A319660 The p-rank of a finite abelian group G is equal to log_p(#{x belongs to G : x^p = 1}) where p is a prime number. In this case, G is the class group of Q(sqrt(-k)), and #{x belongs to G : x^p = 1} is the number of genera of Q(sqrt(-k)) (cf. A003641).
%H A319660 Rick L. Shepherd, <a href="https://libres.uncg.edu/ir/uncg/listing.aspx?id=15057">Binary quadratic forms and genus theory</a>, Master of Arts Thesis, University of North Carolina at Greensboro, 2013.
%F A319660 a(n) = log_2(A003641(n)) = omega(A039957(n)) - 1, where omega(k) is the number of distinct prime divisors of k.
%o A319660 (PARI) for(n=1, 1000, if(n%4==3 && issquarefree(n), print1(omega(n) - 1, ", ")))
%Y A319660 Cf. A003641, A039957, A319659, A319661.
%K A319660 nonn
%O A319660 1,41
%A A319660 _Jianing Song_, Sep 25 2018