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 A123563 #9 Feb 16 2025 08:33:03 %S A123563 455,615,776,824,836,920,1064,1124,1160,1263,1284,1460,1495,1524,1544, %T A123563 1592,1604,1652,1695,1739,1748,1796,1880,1887,1896,1928,1940,1956, %U A123563 2136,2247,2360,2404,2407,2483,2487,2532,2552 %N A123563 Discriminants of imaginary quadratic fields with class number 20 (negated). %C A123563 A finite sequence with exactly 350 terms. %H A123563 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ClassNumber.html">Class Number.</a> %t A123563 Reap[ For[n = 1, n < 3000, n++, s = Sqrt[-n]; If[ NumberFieldClassNumber[s] == 20, d = -NumberFieldDiscriminant[s]; Print[d]; Sow[d]]]][[2, 1]] // Union (* _Jean-François Alcover_, Oct 05 2012 *) %Y A123563 Cf. A006203, A013658, A014602, A014603, A046002, ..., A046016, A046018, A046020. %K A123563 nonn,fini %O A123563 1,1 %A A123563 _Eric W. Weisstein_, Nov 19 2006