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A283362
a(n) = (floor(2*n/3))! mod (2n-1).
1
0, 1, 2, 2, 6, 2, 11, 0, 6, 17, 0, 1, 20, 0, 1, 2, 0, 0, 7, 0, 15, 40, 0, 41, 0, 0, 20, 0, 0, 26, 47, 0, 0, 47, 0, 18, 33, 0, 0, 42, 0, 75, 0, 0, 31, 0, 0, 0, 21, 0, 94, 9, 0, 56, 65, 0, 95, 0, 0, 0, 0, 0, 0, 99, 0, 57, 0, 0, 32, 121, 0, 0, 0, 0, 148, 64, 0, 0, 49
OFFSET
1,3
COMMENTS
if a(n) > 0 then 2n-1, except n=5 and n=13, is prime.
LINKS
MAPLE
f:= proc(n)
local m, r, p, k;
m:= floor(2*n/3);
r:= 2*n-1;
p:= 1;
for k from 1 to m do
p:= p*k mod r;
if p = 0 then break fi;
od:
p
end proc:
f(1):= 0:
map(f, [$1..100]); # Robert Israel, Mar 08 2017
MATHEMATICA
Table[Mod[Floor[(2n)/3]!, 2n-1], {n, 80}] (* Harvey P. Dale, Aug 21 2024 *)
PROG
(PARI) a(n) = (2*n\3)! % (2*n-1); \\ Michel Marcus, Mar 07 2017
CROSSREFS
Sequence in context: A110765 A176991 A264666 * A337524 A324047 A091818
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Michel Marcus, Mar 07 2017
STATUS
approved