OFFSET
1,3
COMMENTS
Discriminant -27.
From Jianing Song, Mar 13 2021: (Start)
Numbers in A003136 that are not congruent to 3 modulo 9.
Closed under multiplication.
For k > 0, k is a term if and only if: write k = 3^a * Product_{i=1..r} (p_i)^(a_i) * Product_{i=1..s} (q_i)^(b_i), p_i == 1 (mod 3), q_i == 2 (mod 3) are primes, then a != 1 and each b_i is even. (End)
LINKS
N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)
MATHEMATICA
Select[Range[0, 350], Resolve@Exists[{x, y}, Reduce[# == (x^2 + x y + 7 y^2), {x, y}, Integers]] &] (* Vincenzo Librandi, Feb 11 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 02 2014
STATUS
approved