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 A005847 Imaginary quadratic fields with class number 2 (a finite sequence). (Formerly M3749) 9
 5, 6, 10, 13, 15, 22, 35, 37, 51, 58, 91, 115, 123, 187, 235, 267, 403, 427 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS n such that Q(sqrt(-n)) has class number 2. 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). REFERENCES 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). P. Ribenboim, The Book of Prime Number Records. Springer-Verlag, NY, 2nd ed., 1989, p. 142. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Steven Arno, M. L. Robinson, Ferrell S. Wheeler, Imaginary quadratic fields with small odd class number, Acta Arith. 83 (1998), pp. 295-330. Keith Matthews, Tables of imaginary quadratic fields with small class numbers. MATHEMATICA Select[Range[200], MoebiusMu[#] != 0 && NumberFieldClassNumber[Sqrt[-#]] == 2 &] (* Alonso del Arte, May 28 2015 *) PROG (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) CROSSREFS Sequence in context: A062845 A236307 A166563 * A109758 A287665 A015820 Adjacent sequences:  A005844 A005845 A005846 * A005848 A005849 A005850 KEYWORD nonn,fini,full AUTHOR STATUS approved

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Last modified September 20 03:49 EDT 2019. Contains 327210 sequences. (Running on oeis4.)