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 A317987 #17 Oct 09 2018 01:48:09 %S A317987 39,55,56,63,68,80,128,136,144,155,156,171,184,196,203,208,219,220, %T A317987 224,252,256,259,260,264,275,276,291,292,308,320,323,328,336,355,360, %U A317987 363,384,387,388,400,456,468,475,504,507,528,544,552,564,568,576,580,592,600,603,612,616,624,640 %N A317987 Discriminants of orders of imaginary quadratic fields with 2 classes per genus, negated. %C A317987 k is a term iff the form class group of positive binary quadratic forms with discriminant -k is isomorphic to (C_2)^r X C_4. %C A317987 This is a subsequence of A133676, so it's finite. It seems that this sequence has 324 terms, the largest being 87360. %C A317987 The smallest number in A133676 but not here is 3600. %H A317987 Jianing Song, <a href="/A317987/b317987.txt">Table of n, a(n) for n = 1..324</a> %H A317987 Rick L. Shepherd, <a href="http://libres.uncg.edu/ir/uncg/f/Shepherd_uncg_0154M_11099.pdf">Binary quadratic forms and genus theory</a>, Master of Arts Thesis, University of North Carolina at Greensboro, 2013. %F A317987 The form class groups of positive binary quadratic forms with discriminant -39, -55, -56, -63, -68, -80 and -128 are all isomorphic to C_4, so 39, 55, 56, 63, 68, 80 and 128 are all members of this sequence. %o A317987 (PARI) isA317987(n) = (-n)%4 < 2 && 2^(1+#quadclassunit(-n)[2])==quadclassunit(-n)[1] %Y A317987 Cf. A003171, A133676. %Y A317987 Fundamental terms are listed in A319983. %K A317987 nonn,fini %O A317987 1,1 %A A317987 _Jianing Song_, Oct 02 2018