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 A319659 #15 Feb 24 2021 08:17:06
%S A319659 0,0,0,0,0,1,0,1,0,1,0,1,1,1,0,0,1,1,1,1,0,0,1,0,0,0,2,1,1,1,1,0,1,0,
%T A319659 1,1,1,1,2,1,0,0,2,1,0,1,1,0,1,1,1,0,1,0,2,0,1,1,1,0,2,0,1,0,1,1,1,0,
%U A319659 0,2,2,1,1,0,1,1,1,0,2,1,2,0,2,1,0,2,2
%N A319659 2-rank of the class group of imaginary quadratic field with discriminant -k, k = A003657(n).
%C A319659 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. A003640).
%H A319659 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 A319659 a(n) = log_2(A003640(n)) = omega(A003657(n)) - 1, where omega(k) is the number of distinct prime divisors of k.
%o A319659 (PARI) for(n=1, 1000, if(isfundamental(-n), print1(omega(n) - 1, ", ")))
%Y A319659 Cf. A003640, A003657, A319660, A319661, A319662.
%Y A319659 Real discriminant case: A317991.
%K A319659 nonn
%O A319659 1,27
%A A319659 _Jianing Song_, Sep 25 2018