A090368 a(1) = 1; for n > 1, smallest divisor > 1 of 2n-1.

1, 3, 5, 7, 3, 11, 13, 3, 17, 19, 3, 23, 5, 3, 29, 31, 3, 5, 37, 3, 41, 43, 3, 47, 7, 3, 53, 5, 3, 59, 61, 3, 5, 67, 3, 71, 73, 3, 7, 79, 3, 83, 5, 3, 89, 7, 3, 5, 97, 3, 101, 103, 3, 107, 109, 3, 113, 5, 3, 7, 11, 3, 5, 127, 3, 131, 7, 3, 137, 139, 3, 11, 5, 3, 149, 151, 3, 5, 157, 3, 7

Offset: 1

Author

Lekraj Beedassy, Nov 27 2003

Keywords

• nonn

Comments

For n > 1, a(n) is an odd prime. - Chai Wah Wu, Nov 14 2022

Links

• Remi Eismann, Table of n, a(n) for n = 1..10000

Crossrefs

Bisection of A020639.

Cf. A100773.

Programs

Mathematica

Join[{1},Divisors[#][[2]]&/@Range[3,161,2]] (* Harvey P. Dale, Oct 09 2011 *)
a[n_] := FactorInteger[2*n-1][[1,1]]; Array[a, 80] (* Amiram Eldar, Jun 11 2022 *)

PARI

a(n)=if(n>1, factor(2*n-1)[1,1], 1) \\ Charles R Greathouse IV, Nov 09 2016

Python

from sympy import primefactors
def A090368(n): return min(primefactors((n<<1)-1), default=1) # Chai Wah Wu, Nov 14 2022

Extensions

More terms from Ray Chandler, Dec 02 2003

Definition corrected by Rémi Eismann, May 08 2007