OFFSET
1,1
COMMENTS
Except for a(1) = 7, these are the primes which can be written in the form a^2 + 7*b^2 with a > 0 and b > 0. - V. Raman, Sep 08 2012
These are the primes p for which p^3 - 1 is divisible by 7, with two exceptions: p = 2 is not in the sequence even though 2^3 - 1 is divisible by 7, and p = 7 is in the sequence even though 7^3 - 1 is not divisible by 7. Except for p = 7, if p^3 - 1 is not divisible by 7, it is congruent to 5 (mod 7). - Richard R. Forberg, Jun 03 2013
REFERENCES
David A. Cox, "Primes of the Form x^2 + n y^2", Wiley, 1989.
LINKS
Ray Chandler, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe)
Sushma Palimar and B. R. Shankar, Mersenne Primes in Real Quadratic Fields, Journal of Integer Sequences, Vol. 15 (2012), #12.5.6.
N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)
FORMULA
Primes congruent to {1, 7, 9, 11, 15, 23, 25} (mod 28). - T. D. Noe, Apr 29 2008
MATHEMATICA
QuadPrimes2[1, 0, 7, 10000] (* see A106856 *)
PROG
(PARI) is(n)=kronecker(n, 7)>=0 && isprime(n) && n>2 \\ Charles R Greathouse IV, Nov 19 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved