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

3,1

Comments

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

Links

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

Example

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

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 *)

Prog

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

Crossrefs

Essentially a combination of A005420 and A002587.

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

Sequence in context: A359115 A287939 A129738 * A292006 A105603 A264987

Adjacent sequences: A271311 A271312 A271313 * A271315 A271316 A271317

Keyword

nonn

Author

Altug Alkan, Apr 03 2016

Status

approved