A271314 Largest prime factor of the n-th Jacobsthal number, A001045(n).

3, 5, 11, 7, 43, 17, 19, 31, 683, 13, 2731, 127, 331, 257, 43691, 73, 174763, 41, 5419, 683, 2796203, 241, 4051, 8191, 87211, 127, 3033169, 331, 715827883, 65537, 20857, 131071, 86171, 109, 25781083, 524287, 22366891, 61681, 8831418697, 5419, 2932031007403, 2113, 18837001

Offset: 3

Author

Altug Alkan, Apr 03 2016

Keywords

• nonn

Comments

a(22) = 683 is the first repeated term in this sequence. Note that a(n+2) = A129738(n), for n < 20.

Examples

			a(6) = 7 because A001045(6) = 21 = 3*7.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 3..1122 (terms 3..988 from Charles R Greathouse IV)

Crossrefs

Essentially a combination of A005420 and A002587.

Cf. A001045, A006530, A060385, A129738, A264137.

Programs

Mathematica

FactorInteger[#][[-1, 1]] & /@ Take[#, -(Length@ # - 3)] &@ CoefficientList[Series[x/(1 - x - 2 x^2), {x, 0, 45}], x] (* Michael De Vlieger, Apr 04 2016, after Robert G. Wilson v at A001045 *)

PARI

a001045(n) = (2^n - (-1)^n) / 3;
a(n) = vecmax(factor(a001045(n))[,1]);