OFFSET
1,3
COMMENTS
Also numbers representable as x^2 + 5*x*y + y^2 with 0 <= x <= y. - Gheorghe Coserea, Jul 29 2018
Also numbers of the form x^2 - x*y - 5*y^2 with 0 <= x <= y (or x^2 + x*y - 5*y^2 with x, y nonnegative). - Jianing Song, Jul 31 2018
Also nonnegative numbers of the form 7*x^2 - 3*y^2. - Jon E. Schoenfield, Jun 03 2022
LINKS
Gheorghe Coserea, Table of n, a(n) for n = 1..100001
N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)
MATHEMATICA
A={}; For[ n=0, n <= 300, n++,
If[ Length[ Reduce[x^2 + 3 x y - 3 y^2 - n == 0, {x, y}, Integers]]>0, AppendTo[A, n]]]; A
PROG
(PARI)
\\ From Bill Allombert, Jun 04 2014. Since 21 is a fundamental discriminant, and the polynomial is unitary, the following code works:
B=bnfinit(x^2+3*x-3); select(n->#bnfisintnorm(B, n), [1..500])
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 01 2014
STATUS
approved