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 A005847 M3749 #35 Apr 17 2025 12:50:58
%S A005847 5,6,10,13,15,22,35,37,51,58,91,115,123,187,235,267,403,427
%N A005847 Imaginary quadratic fields with class number 2 (a finite sequence).
%C A005847 n such that Q(sqrt(-n)) has class number 2.
%C A005847 The PARI code lists the imaginary quadratic fields Q(sqrt(-d)) with small class number and produces A003173 (class number 1), A005847 (2), A006203 (3).
%D A005847 J. M. Masley, Where are the number fields with small class number?, pp. 221-242 of "Number Theory, Carbondale 1979", Lect. Notes Math. 751 (1982).
%D A005847 Paulo Ribenboim, The Book of Prime Number Records. Springer-Verlag, NY, 2nd ed., 1989, p. 142.
%D A005847 Paulo Ribenboim, The Little Book of Bigger Primes, Springer-Verlag NY 2004. See pp. 143-144.
%D A005847 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A005847 Steven Arno, M. L. Robinson, and 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), pp. 295-330.
%H A005847 David Masser, <a href="https://arxiv.org/abs/2010.10256">Alan Baker</a>, arXiv:2010.10256 [math.HO], 2020. See p. 24.
%H A005847 Keith Matthews, <a href="http://www.numbertheory.org/classnos/">Tables of imaginary quadratic fields with small class numbers</a>.
%H A005847 <a href="/index/Qua#quadfield">Index entries for sequences related to quadratic fields</a>.
%t A005847 Select[Range[200], MoebiusMu[#] != 0 && NumberFieldClassNumber[Sqrt[-#]] == 2 &] (* _Alonso del Arte_, May 28 2015 *)
%o A005847 (PARI) { bnd = 10000; S = vector(10,X,[]); for (i = 1, bnd, if (issquarefree(i), n = qfbclassno(if(i%4==3,-i,-4*i)); if (n<11, S[n] = concat(S[n],i), ), )); } \\ Robert Harley (Robert.Harley(AT)inria.fr)
%Y A005847 Cf. A003173, A005847, A006203.
%K A005847 nonn,fini,full
%O A005847 1,1
%A A005847 _N. J. A. Sloane_