

A003656


Discriminants of real quadratic fields with unique factorization.
(Formerly M3777)


28



5, 8, 12, 13, 17, 21, 24, 28, 29, 33, 37, 41, 44, 53, 56, 57, 61, 69, 73, 76, 77, 88, 89, 92, 93, 97, 101, 109, 113, 124, 129, 133, 137, 141, 149, 152, 157, 161, 172, 173, 177, 181, 184, 188, 193, 197, 201, 209, 213, 217, 233, 236, 237, 241, 248, 249, 253, 268, 269
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OFFSET

1,1


COMMENTS

Discriminants of real quadratic fields with class number 1.
Other than the term 8, every term is of one of the three following forms: (i) p, where p is a prime congruent to 1 modulo 4; (ii) 4p or 8p, where p is a prime congruent to 3 modulo 4; (iii) pq, where p, q are distinct primes congruent to 3 modulo 4. In fact, for a positive fundamental discriminant d, the class number of the real quadratic field of discriminant d is odd if and only if d = 8 or is of the form (i), (ii) or (iii). See Theorem 1 and Theorem 2 of Ezra Brown's link.  Jianing Song, Feb 24 2021


REFERENCES

D. A. Buell, Binary Quadratic Forms. SpringerVerlag, NY, 1989, pp. 224241.
H. Cohen, Advanced Topics in Computational Number Theory, Springer, 2000, p. 534.
H. Hasse, Number Theory, SpringerVerlag, NY, 1980, p. 576.
Pohst and Zassenhaus, Algorithmic Algebraic Number Theory, Cambridge Univ. Press, page 432.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000
Ezra Brown, Class numbers of real quadratic number fields, Trans. Amer. Math. Soc. 190 (1974), 99107.
Henri Cohen and X.F. Roblot, Computing the Hilbert Class Field of Real Quadratic Fields, Math. Comp. 69 (2000), 12291244.
Eric Weisstein's World of Mathematics, Class Number
Index entries for sequences related to quadratic fields


MATHEMATICA

maxDisc = 269; t = Table[ {NumberFieldDiscriminant[ Sqrt[n] ], NumberFieldClassNumber[ Sqrt[n] ]}, {n, Select[ Range[2, maxDisc], SquareFreeQ] } ]; Union[ Select[ t, #[[2]] == 1 && #[[1]] <= maxDisc & ][[All, 1]]] (* JeanFrançois Alcover, Jan 24 2012 *)


PROG

(Sage)
is_fund_and_qfbcn_1 = lambda n: is_fundamental_discriminant(n) and QuadraticField(n, 'a').class_number() == 1
A003656 = lambda n: filter(is_fund_and_qfbcn_1, (1, 2, .., n))
A003656(270) # Peter Luschny, Aug 10 2014


CROSSREFS

Cf. A003652, A003658, A014602 (imaginary case).
For discriminants of real quadratic number fields with class number 2, 3, ..., 10, see A094619, A094612A094614, A218156A218160; see also A035120.
Sequence in context: A079896 A133315 A003658 * A003246 A143748 A124378
Adjacent sequences: A003653 A003654 A003655 * A003657 A003658 A003659


KEYWORD

nonn,nice


AUTHOR

N. J. A. Sloane, Mira Bernstein


EXTENSIONS

More terms from Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), May 15 2002


STATUS

approved



