A172255 Partial sums of the Fermat pseudoprimes to base 2, A001567.

341, 902, 1547, 2652, 4039, 5768, 7673, 9720, 12185, 14886, 17707, 20984, 25017, 29386, 33757, 38438, 43899, 50500, 58457, 66778, 75259, 84170, 94431, 105016, 116321, 129122, 142863, 156610, 170591, 185082, 200791, 216632, 233337, 252042

Offset: 1

Author

Jonathan Vos Post, Jan 29 2010

Keywords

• easy
• nonn

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.

Examples

			a(15) = 341 + 561 + 645 + 1105 + 1387 + 1729 + 1905 + 2047 + 2465 + 2701 + 2821 + 3277 + 4033 + 4369 + 4371 = 33757 is prime.
		

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000040, A001567.

Programs

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

Formula

a(n) = Sum_{i=1..n, odd composite numbers n such that 2^(n-1) == 1 mod n}.