OFFSET
1,3
COMMENTS
For n>1, if a(n) > 0 then 2n-1 is prime.
From Robert G. Wilson v, Mar 15 2017: (Start)
Except for n=1, a(n)=0 iff 2n-1 is not prime (A104275).
a(n) is prime for n: 3, 6, 22 & 31. (End)
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
MATHEMATICA
Table[Product[Mod[(2 n - 1), (2 k - 1)], {k, 2, Floor[Sqrt[2 n - 1]/2] + 1}], {n, 80}] (* Michael De Vlieger, Mar 15 2017 *)
PROG
(PARI) a(n)=my(t=2*n-1); prod(k=2, sqrtint(t\4)+1, t%(2*k-1)) \\ Charles R Greathouse IV, Mar 22 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhandos Mambetaliyev, Mar 11 2017
STATUS
approved