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 A172255 Partial sums of the Fermat pseudoprimes to base 2, A001567. 1
 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 (list; graph; refs; listen; history; text; internal format)
 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: A216170 A175736 A179839 * A087835 A271221 A066488 Adjacent sequences:  A172252 A172253 A172254 * A172256 A172257 A172258 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Jan 29 2010 STATUS approved

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