OFFSET
1,1
COMMENTS
The subsequence of pseudoprimes in this sequence begins 341; the next term exceeds a(10000) if it exists. - Charles R Greathouse IV, Aug 22 2012
The subsequence of primes in the sequence begins 7673, 17707, 33757, 270763, 484621.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = SUM[i=1..n] {odd composite numbers n such that 2^(n-1) == 1 mod n}.
EXAMPLE
a(15) = 341 + 561 + 645 + 1105 + 1387 + 1729 + 1905 + 2047 + 2465 + 2701 + 2821 + 3277 + 4033 + 4369 + 4371 = 33757 is prime.
PROG
(PARI) sums(v)=my(s); vector(#v, i, s+=v[i])
sums(select(n->Mod(2, n)^n==2 & !isprime(n), vector(10^5, n, 2*n+1))) \\ Charles R Greathouse IV, Jul 09 2015
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Jan 29 2010
STATUS
approved