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A306989 Numbers not of the form x^2+2*y^2+7*z^2. 1
5, 14, 20, 21, 35, 42, 70, 84, 91, 119, 133, 140, 158, 168, 182, 189, 217, 231, 238, 245, 266, 280, 287, 315, 329, 336, 364, 378, 385, 413, 427, 434, 462, 476, 483, 511, 518, 525, 532, 560, 574, 581, 609, 623, 630, 658, 672, 679, 686, 707, 721, 728, 756, 770 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
It appears that the only terms not divisible by 7 are 5, 29 and 158. - Robert Israel, Mar 27 2019
LINKS
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 to floor(sqrt(N)) do
for y from 0 to floor(sqrt((N-x^2)/2)) do
for z from 0 do
v:= x^2 + 2*y^2 + 7*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
Sequence in context: A022139 A277351 A143707 * A003248 A353974 A071317
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Mar 26 2019
STATUS
approved

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Last modified May 21 19:35 EDT 2024. Contains 372738 sequences. (Running on oeis4.)