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%I #25 Dec 25 2018 11:29:35
%S 431,503,743,863,1931,2503,2579,2767,2819,3011,3371,4283,4523,4691,
%T 5011,5647,5851,5867,6323,6691,7907,8059,8123,8171,8243,8387,8627,
%U 8747,9091,9187,9811,9859,10067,10771,11731,12107,12547,13171,13291
%N Discriminants of imaginary quadratic fields with class number 21 (negated).
%C 85 discriminants in this sequence (proved).
%H Giovanni Resta, <a href="/A046018/b046018.txt">Table of n, a(n) for n = 1..85</a> (full sequence, from Weisstein's World of Mathematics)
%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="https://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="https://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>
%t Reap[ For[n = 1, n < 14000, n++, s = Sqrt[-n]; If[ NumberFieldClassNumber[s] == 21, d = -NumberFieldDiscriminant[s]; Print[d]; Sow[d]]]][[2, 1]] // Union (* _Jean-François Alcover_, Oct 05 2012 *)
%Y Cf. A006203, A013658, A014602, A014603, A046002 - A046020.
%K nonn,fini,full
%O 1,1
%A _Eric W. Weisstein_
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