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 A046018 #26 Feb 16 2025 08:32:38 %S A046018 431,503,743,863,1931,2503,2579,2767,2819,3011,3371,4283,4523,4691, %T A046018 5011,5647,5851,5867,6323,6691,7907,8059,8123,8171,8243,8387,8627, %U A046018 8747,9091,9187,9811,9859,10067,10771,11731,12107,12547,13171,13291 %N A046018 Discriminants of imaginary quadratic fields with class number 21 (negated). %C A046018 85 discriminants in this sequence (proved). %H A046018 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 A046018 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 A046018 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 A046018 Keith Matthews, <a href="http://www.numbertheory.org/classnos/">Tables of imaginary quadratic fields with small class numbers</a> %H A046018 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 A046018 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ClassNumber.html">Class Number.</a> %t A046018 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 A046018 Cf. A006203, A013658, A014602, A014603, A046002 - A046020. %K A046018 nonn,fini,full %O A046018 1,1 %A A046018 _Eric W. Weisstein_