login
A271314
Largest prime factor of the n-th Jacobsthal number, A001045(n).
2
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.
Sequence in context: A359115 A287939 A129738 * A292006 A105603 A264987
KEYWORD
nonn
AUTHOR
Altug Alkan, Apr 03 2016
STATUS
approved