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A003640
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Number of genera of imaginary quadratic field with discriminant -k, k = A003657(n).
(Formerly M0090)
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6
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1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 2, 2, 2, 1, 1, 2, 2, 2, 2, 1, 1, 2, 1, 1, 1, 4, 2, 2, 2, 2, 1, 2, 1, 2, 2, 2, 2, 4, 2, 1, 1, 4, 2, 1, 2, 2, 1, 2, 2, 2, 1, 2, 1, 4, 1, 2, 2, 2, 1, 4, 1, 2, 1, 2, 2, 2, 1, 1, 4, 4, 2, 2, 1, 2, 2, 2, 1, 4, 2, 4, 1, 4, 2, 1, 4, 4, 1, 2, 2, 2, 2, 2, 2, 2, 1, 4, 1, 4, 2, 2, 2, 2, 1, 2
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OFFSET
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1,6
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COMMENTS
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The number of genera of a quadratic field is equal to the number of elements x in the class group such that x^2 = e where e is the identity. - Jianing Song, Jul 24 2018
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REFERENCES
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D. A. Buell, Binary Quadratic Forms. Springer-Verlag, NY, 1989, pp. 224-241.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = 2^(t-1) where t = number of distinct prime divisors of A003657(n).
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MATHEMATICA
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okQ[n_] := n != 1 && SquareFreeQ[n/2^IntegerExponent[n, 2]] && (Mod[n, 4] == 3 || Mod[n, 16] == 8 || Mod[n, 16] == 4);
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PROG
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(PARI) for(n=1, 1000, if(isfundamental(-n), print1(2^(omega(n) - 1), ", "))) \\ Andrew Howroyd, Jul 24 2018
(PARI) for(n=1, 1000, if(isfundamental(-n), print1(2^#select(t->t%2==0, quadclassunit(-n).cyc), ", "))) \\ Andrew Howroyd, Jul 24 2018
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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Name clarified and offset corrected by Jianing Song, Jul 24 2018
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STATUS
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approved
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