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

1,1

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

Cf. A000040, A001567.

Sequence in context: A175736 A372896 A179839 * A087835 A271221 A066488

Adjacent sequences: A172252 A172253 A172254 * A172256 A172257 A172258

Keyword

easy,nonn

Author

Jonathan Vos Post, Jan 29 2010

Status

approved