OFFSET
1,1
COMMENTS
Name was: Primes of the form x^2 + 3*x*y - 3*y^2 (as well as of the form x^2 + 5*x*y + y^2).
Discriminant = 21. Class number = 2. Binary quadratic forms a*x^2 + b*x*y + c*y^2 have discriminant d = b^2-4ac and gcd(a,b,c)=1 (primitive).
Primes of the form 6n+1 which cannot be expressed as 7k-1, 7k-2, or 7k-4. a(n)^2 == 1 (mod 24). - Gary Detlefs, Jan 26 2014
Besides 7 (which divides 21), primes of the form p == 1 (mod 3) and either == 1 or 2 or 4 (mod 7). For the other class, the primes represented by the principal form [3, 3, -1] (or primitive forms equivalent to this) are besides 3 (which divides 21), congruent to 2 (mod 3) and also to 3, 5, 6 (mod 7). For the primes of both classes see A038893. - Wolfdieter Lang, Jun 19 2019
REFERENCES
Z. I. Borevich and I. R. Shafarevich, Number Theory.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Peter Luschny, Binary Quadratic Forms
N. J. A. Sloane et al., Binary Quadratic Forms and OEIS: Index to related sequences, programs, references. OEIS wiki, June 2014.
D. B. Zagier, Zetafunktionen und quadratische Körper, Springer, 1981.
EXAMPLE
a(1)=7 because we can write 7 = 2^2 + 3*2*1 - 3*1^2 (or 7 = 1^2 + 5*1*1 + 1^2).
MAPLE
f:=n->7*ceil((6*n+1)/7)-(6*n+1):for n from 1 to 220 do if isprime(6*n+1) and f(n)<>1 and f(n)<>2 and f(n)<>4 then print(6*n+1) fi od. # Gary Detlefs, Jan 26 2014
MATHEMATICA
xy[{x_, y_}]:={x^2+3x y-3y^2, y^2+3x y -3x^2}; Union[Select[Flatten[xy/@ Subsets[ Range[50], {2}]], #>0&&PrimeQ[#]&]] (* Harvey P. Dale, Feb 17 2013 *)
PROG
(Sage) # uses[binaryQF]
# The function binaryQF is defined in the link 'Binary Quadratic Forms'.
Q = binaryQF([1, 3, -3])
Q.represented_positives(1326, 'prime') # Peter Luschny, Jun 24 2019
KEYWORD
dead
AUTHOR
Laura Caballero Fernandez, Lourdes Calvo Moguer, Maria Josefa Cano Marquez, Oscar Jesus Falcon Ganfornina and Sergio Garrido Morales (laucabfer(AT)alum.us.es), Jun 12 2008
EXTENSIONS
More terms from Harvey P. Dale, Feb 17 2013
STATUS
approved