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 A051998 #10 Sep 21 2021 15:47:28 %S A051998 1,-15,-5,-17,-19,-21,1,-3,1,1,5,3,-33,1,-35,-5,5,-39,-13,-41,3,1,1, %T A051998 -15,-3,3,5,15,-7,-51,1,3,-55,1,-5,5,-3,1,-21,-65,5,13,-69,3,-15,5, %U A051998 -11,7,3,-17,17,-89,29,-91,13,-95,-5,33,1,-15,35,39,11,5,1,3,19,5,-11,5,-51,5,53,-165,-7,-3,1,21,-29,-5,21 %N A051998 Consider real quadratic fields of ERD-type with class groups of exponent 2 and discriminants of the form D = r^2*k^2+4k, k odd; sequence gives values of k. %C A051998 Excludes discriminants appearing in A051990. %D A051998 R. A. Mollin, Quadratics, CRC Press, 1996, Appendix A, Table A4(3), column k. %Y A051998 Cf. A051990, A051991, A051992, A051993, A051994, A051995, A051996, A051997. %K A051998 sign,fini,full %O A051998 1,2 %A A051998 _N. J. A. Sloane_, Jan 04 2000 %E A051998 a(69) and following from _Georg Fischer_, Sep 21 2021