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 A001991 M0212 N0863 #31 Nov 19 2022 04:39:13
%S A001991 2,2,2,2,2,2,2,2,46,46,46,46,46,46,406,718,950,950,950,1698,1698,1698,
%T A001991 1698,1698,3990,3990,3990,53510,77970,89478,89478,89478,89478,89478,
%U A001991 89478
%N A001991 Class numbers associated with terms of A001990.
%D A001991 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A001991 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A001991 D. H. Lehmer, E. Lehmer and D. Shanks, <a href="https://doi.org/10.1090/S0025-5718-1970-0271006-X">Integer sequences having prescribed quadratic character</a>, Math. Comp., 24 (1970), 433-451.
%H A001991 D. H. Lehmer, E. Lehmer and D. Shanks, <a href="/A002189/a002189.pdf">Integer sequences having prescribed quadratic character</a>, Math. Comp., 24 (1970), 433-451 [Annotated scanned copy]
%H A001991 <a href="/index/Qua#quadfield">Index entries for sequences related to quadratic fields</a>
%F A001991 a(n) = ClassNumber(-8*A001990(n)). - _Sean A. Irvine_, Mar 06 2013
%o A001991 (PARI) isok(p, oddpn) = {forprime(q=3, oddpn, if (kronecker(p, q) != -kronecker(-2, q), return (0)); ); return (1); }
%o A001991 a(n) = {oddpn = prime(n+1); forprime(p=3, , if (((p%8) == 5) && isok(p, oddpn), return (qfbclassno(-8*p))); ); } \\ _Michel Marcus_, Oct 19 2017
%Y A001991 Cf. A001990.
%K A001991 nonn,more
%O A001991 1,1
%A A001991 _N. J. A. Sloane_
%E A001991 Better name from _Sean A. Irvine_, Mar 06 2013
%E A001991 Offset changed by _Michel Marcus_, Oct 19 2017