

A003173


Heegner numbers: imaginary quadratic fields with unique factorization (or class number 1).
(Formerly M0827)


34




OFFSET

1,2


COMMENTS

Could also be called Gauss numbers, since he discovered them. Heegner proved list is complete.  Artur Jasinski, Mar 21 2003
Numbers n such that Q(sqrt(n)) has unique factorization into primes.
These are the squarefree values of n for which if some positive integer N can be written in the form (a/2)^2+n*(b/2)^2 for integers a and b, then every prime factor P of N which occurs to an odd power can also be written in the form (c/2)^2+n*(d/2)^2 for integers c and d.  V. Raman, Sep 17 2012, May 01 2013
For n = 1 and n = 2, the rings Z[i] (Gaussian Integers), and Z(sqrt(2)) = numbers of the form a + b*sqrt(2), where a and b are integers, admit unique factorization.  V. Raman, Sep 17 2012
For the values of n congruent to 3 (mod 4), the set of numbers of the form (a/2) + (b/2)*sqrt(n), for integers a and b of the same parity, admit unique factorization.  V. Raman, Sep 17 2012, corrected by Eric M. Schmidt, Feb 17 2013


REFERENCES

J. H. Conway and R. K. Guy, The Book of Numbers, Copernicus Press, NY, 1996, p. 224.
N. Elkies, The Klein quartic in number theory, pp. 51101 of S. Levy, ed., The Eightfold Way, Cambridge Univ. Press, 1999. MR1722413 (2001a:11103). See page 93.
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 5th ed., Oxford Univ. Press, 1979, p. 213.
Heegner K., 1952. Diophantische Analysis und Modulfunktionen. Matematische Zeitschrift Vol. 56 p. 227253.
W. W. J. Hulsbergen, Conjectures in Arithmetic Algebraic Geometry, Vieweg, 1994, p. 8.
J. M. Masley, Where are the number fields with small class number?, pp. 221242 of Number Theory Carbondale 1979, Lect. Notes Math. 751 (1982).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
H. M. Stark, An Introduction to Number Theory. Markham, Chicago, 1970, p. 295.


LINKS

Table of n, a(n) for n=1..9.
YangHui He, John McKay, Sporadic and Exceptional, arXiv:1505.06742 [math.AG], 2015.
Rick L. Shepherd, Binary quadratic forms and genus theory, Master of Arts Thesis, University of North Carolina at Greensboro, 2013.
Eric Weisstein's World of Mathematics, Gauss's Class Number Problem [Yes, 3 s's in that URL]
Eric Weisstein's World of Mathematics, Heegner Number
Wikipedia, Heegner number
Index entries for sequences related to quadratic fields


FORMULA

a(n) = A263465(n) = A048981(6n) for n = 1, 2, 3, 4, 5.  Jonathan Sondow, Dec 09 2015
From Alexander R. Povolotsky, Dec 26 2015: (Start)
a(n) = 1+((1 + sqrt(3))^(n1)  (1  sqrt(3))^(n1))/(2*sqrt(3)) for n = 1,2,3,4.
a(n) = 19+24*((1 + sqrt(3))^(n6)  (1  sqrt(3))^(n6))/(2*sqrt(3)) for n = 6,7,8,9; so four almost integers (including one made famous by Ramanujan) could be expressed as: exp(Pi*sqrt(19+24*((1 + sqrt(3))^(n6)  (1  sqrt(3))^(n6))/(2*sqrt(3)))) for n = 6,7,8,9.
In general a(n) = a(k) + (a(k+1)a(k))*((1 + sqrt(3))^(nk)  (1  sqrt(3))^(nk))/(2*sqrt(3)) where k=1 for n = 1,2,3,4 k=1 and k=6 for n = 6,7,8,9. (End)


MATHEMATICA

Union[ Select[ NumberFieldDiscriminant[ Sqrt[#]]& /@ Range[200], NumberFieldClassNumber[ Sqrt[#]] == 1 & ] /. {4 > 1, 8 > 2}] (* JeanFrançois Alcover, Jan 04 2012 *)


PROG

(PARI) select(n>qfbclassno(n*if(n%4==3, 1, 4))==1, vector(200, i, i)) \\ Charles R Greathouse IV, Nov 20 2012


CROSSREFS

Cf. A014602 (for discriminants of these fields), A005847 (for class number 2), A048981, A003174, A263465.
Partitions into Heegner numbers: A242216, A242217.
Sequence in context: A158709 A180422 A055502 * A159262 A160434 A139630
Adjacent sequences: A003170 A003171 A003172 * A003174 A003175 A003176


KEYWORD

fini,nonn,full,nice


AUTHOR

N. J. A. Sloane


STATUS

approved



