OFFSET
1,1
COMMENTS
It appears that the only terms not divisible by 7 are 10, 15, 30, 39, 79, 94, 130, 359, 382, 391, 754, and 1546. - Robert Israel, Mar 27 2019
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
Irving Kaplansky, The first nontrivial genus of positive definite ternary forms, Mathematics of Computation, Vol. 64 (1995): 341-345.
MAPLE
N:= 1000: # for terms <= N
V:= Array(0..N):
for x from 0 while x^2<=N do
for y from 0 while x^2 + 2*y^2 <= N do
for z from 0 do
v:= x^2 + 2*y^2 + 2*y*z + 4*z^2;
if v > N then break fi;
V[v]:= 1;
od od od:
select(t -> V[t]=0, [$1..N]); # Robert Israel, Mar 27 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Mar 26 2019
EXTENSIONS
Thanks to Robert Israel for pointing out that I neglected to mention x, y, z >= 0. - N. J. A. Sloane, Mar 27 2019
STATUS
approved