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A227735 Negative fundamental discriminants with cyclic class groups of composite order (negated). 1
39, 55, 56, 68, 87, 95, 104, 111, 116, 119, 136, 143, 152, 155, 159, 164, 183, 184, 199, 203, 212, 215, 219, 239, 244, 247, 248, 259, 287, 291, 292, 295, 296, 299, 303, 319, 323, 327, 328, 335, 339, 344, 355, 356, 367, 371, 376, 388, 391, 395, 404, 407, 411 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Absolute values of fundamental discriminants of imaginary quadratic fields whose class groups are cyclic of composite order. Of course every class group of squarefree order is necessarily cyclic. (This means the negatives of negative fundamental discriminants with class groups of composite squarefree orders are a proper subsequence.)

The n-th line of the linked file gives the order of the class group (the class number) corresponding to the fundamental discriminant -a(n).

The negative of each term is either a negative fundamental discriminant or the product of exactly one positive prime discriminant and one negative prime discriminant where the product contains at most one factor in {-8, -4, 8} and is unique disregarding order. In the first case, the class number is odd (because the 2-rank of the class group is 0); in the second, even (because the 2-rank is 1).

LINKS

Rick L. Shepherd, Table of n, a(n) for n = 1..10000

Rick L. Shepherd, Binary quadratic forms and genus theory, Master of Arts Thesis, University of North Carolina at Greensboro, 2013.

Rick L. Shepherd, Orders of corresponding class groups

Index entries for sequences related to quadratic fields

EXAMPLE

The fundamental discriminant -39 = (-3)(13) has a cyclic class group of order 4, which is composite (but not squarefree). The fundamental discriminant -104 = (-8)(13) has a cyclic class group of order 6, which is composite. The fundamental discriminant -239 is itself a prime discriminant with cyclic class group of order 15, also composite (but not divisible by 2).

PROG

(PARI)

{default(realprecision, 100);

terms_wanted = 10000;

t = 0; k = 0;

while(t < terms_wanted,

  k++;

  if(isfundamental(-k),

    F = bnfinit(quadpoly(-k, x), , [6, 6, 4]);

    if(bnfcertify(F) <> 1,

      print("Certify failed for ", -k, " -- exiting (",

        t, " terms found)"); break);

    if(length(F.clgp.cyc) == 1 &&

      isprime(F.clgp.cyc[1]) == 0,

      t++;

      write("b227735.txt", t, " ", k);

      write("a227735.txt", t, " ", F.clgp.cyc[1]))))}

CROSSREFS

Cf. A227734.

Sequence in context: A317987 A330219 A013658 * A319983 A165346 A268083

Adjacent sequences:  A227732 A227733 A227734 * A227736 A227737 A227738

KEYWORD

nonn

AUTHOR

Rick L. Shepherd, Jul 29 2013

STATUS

approved

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Last modified August 11 15:12 EDT 2020. Contains 336428 sequences. (Running on oeis4.)