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%I #28 Jul 08 2015 10:04:09
%S 647,1039,1103,1279,1447,1471,1811,1979,2411,2671,3491,3539,3847,3923,
%T 4211,4783,5387,5507,5531,6563,6659,6703,7043,9587,9931,10867,10883,
%U 12203,12739,13099,13187,15307,15451,16267,17203,17851,18379,20323
%N Discriminants of imaginary quadratic fields with class number 23 (negated).
%H Giovanni Resta, <a href="/A046020/b046020.txt">Table of n, a(n) for n = 1..68</a> (full sequence, from Steven Arno et al.)
%H Steven Arno, M. L. Robinson, Ferrell S. Wheeler, <a href="http://matwbn.icm.edu.pl/ksiazki/aa/aa83/aa8341.pdf">Imaginary quadratic fields with small odd class number</a>, Acta Arith. 83 (1998) 295-330.
%H Duncan A. Buell, <a href="http://dx.doi.org/10.1090/S0025-5718-1977-0439802-X">Small class numbers and extreme values of L-functions of quadratic fields</a>, Math. Comp., 31 (1977), 786-796.
%H Keith Matthews, <a href="http://www.numbertheory.org/classnos/">Tables of imaginary quadratic fields with small class numbers</a>
%H C. Wagner, <a href="http://dx.doi.org/10.1090/S0025-5718-96-00722-3">Class Number 5, 6 and 7</a>, Math. Comput. 65, 785-800, 1996.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ClassNumber.html">Class Number.</a>
%H <a href="/index/Qua#quadfield">Index entries for sequences related to quadratic fields</a>
%t Reap[ Do[ If[ NumberFieldClassNumber[ Sqrt[-n] ] == 23, d = -NumberFieldDiscriminant[ Sqrt[-n] ]; Print[d]; Sow[d]], {n, 1, 21000}]][[2, 1]] // Union (* _Jean-François Alcover_, Oct 22 2012 *)
%o (PARI) select(n->qfbclassno(-n)==23, vector(22696, n, 4*n+3)) \\ _Charles R Greathouse IV_, Apr 25 2013
%Y Cf. A006203, A013658, A014602, A014603, A046002-A046020.
%K nonn,fini,full
%O 1,1
%A _Eric W. Weisstein_
%E 68 discriminants in this sequence (proved).
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