A001991 - OEIS

%I M0212 N0863 #31 Nov 19 2022 04:39:13

%S 2,2,2,2,2,2,2,2,46,46,46,46,46,46,406,718,950,950,950,1698,1698,1698,

%T 1698,1698,3990,3990,3990,53510,77970,89478,89478,89478,89478,89478,

%U 89478

%N Class numbers associated with terms of A001990.

%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H 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 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 <a href="/index/Qua#quadfield">Index entries for sequences related to quadratic fields</a>

%F a(n) = ClassNumber(-8*A001990(n)). - _Sean A. Irvine_, Mar 06 2013

%o (PARI) isok(p, oddpn) = {forprime(q=3, oddpn, if (kronecker(p, q) != -kronecker(-2, q), return (0)); ); return (1); }

%o 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 Cf. A001990.

%K nonn,more

%O 1,1

%A _N. J. A. Sloane_

%E Better name from _Sean A. Irvine_, Mar 06 2013

%E Offset changed by _Michel Marcus_, Oct 19 2017