A121170 - OEIS

%I #13 Aug 01 2024 16:48:21

%S 5,11,61,41,3001,61,141961,2161,109441,3001,474541,2521,14736206161,

%T 141961,230686501,3041,3415914041,109441,67735001,570601,8288823481,

%U 474541,2441738887963981,20641,158414167964045700001,14736206161,1114769954367361,12317523121,349619996930737079890201

%N Largest prime divisor of Fibonacci(5n).

%C Except for a(1) = 5 all a(n) are congruent to 1 (mod 10) (final digit is 1). Final digit of most prime divisors of F(5n) is 1.

%H Amiram Eldar, <a href="/A121170/b121170.txt">Table of n, a(n) for n = 1..284</a>

%F a(n) = A060385(5*n).

%e a(2) = 11 because F(10)= 5 * 11.

%e a(4) = 41 because F(20)= 3 * 5 * 11 * 41.

%e a(10) = 3001 because F(50)= 5^2 * 11 * 101 * 151 * 3001.

%e a(25) = 158414167964045700001 because F(125)= 5^3 * 3001 * 158414167964045700001.

%t Table[Max[Flatten[FactorInteger[Fibonacci[5n]]]],{n,1,50}]

%Y Cf. A000045, A060385.

%K nonn

%O 1,1

%A _Alexander Adamchuk_, Aug 14 2006

%E a(27)-a(29) from _Amiram Eldar_, Aug 01 2024