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 A001989 M3756 N1535 #28 Nov 19 2022 04:39:02
%S A001989 1,1,5,7,7,7,9,53,73,83,83,83,157,185,185,185,185,1927,2295,2273,5313,
%T A001989 5313,7173,9529,18545,18545,18545,18545,22635,22635,66011,121725,
%U A001989 344909,344909
%N A001989 Class numbers associated with terms of A001988.
%D A001989 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A001989 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A001989 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 A001989 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]
%o A001989 (PARI) isok(p, oddpn) = {forprime(q=3, oddpn, if (kronecker(p, q) != -kronecker(-1, q), return (0)); ); return (1); }
%o A001989 a(n) = {my(oddpn = prime(n+1)); forprime(p=3, , if (((p%8) == 7) && isok(p, oddpn), return (qfbclassno(-p*if(p%4>1, 4, 1)))););} \\ _Michel Marcus_, Oct 19 2017
%Y A001989 Cf. A001988.
%K A001989 nonn
%O A001989 1,3
%A A001989 _N. J. A. Sloane_
%E A001989 Better name from _Sean A. Irvine_, Mar 06 2013
%E A001989 Terms corrected by _Sean A. Irvine_, Mar 06 2013
%E A001989 Offset changed by _Michel Marcus_, Oct 19 2017