OFFSET
1,1
COMMENTS
Faye et al. prove that no term is of the form 5^k-1. - Michel Marcus, Jun 16 2024
LINKS
Max Alekseyev, Table of n, a(n) for n = 1..502
Bernadette Faye, Florian Luca, and Amadou Tall, On the equation phi(5^m-1)=5^n-1, Bull. Korean Math. Soc. 2015; 52(2): 513-524.
Eric Weisstein's World of Mathematics, Totient Function
Wikipedia, Euler's totient function
FORMULA
a(n) = n*A027741(n).
MATHEMATICA
EulerPhi[5^Range[25] - 1] (* Paolo Xausa, Jun 18 2024 *)
PROG
(PARI) {a(n) = eulerphi(5^n-1)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Nov 22 2017
STATUS
approved