OFFSET
1,1
COMMENTS
a(n) == 0 (mod 25).
a(n) == {0, 25, 75} (mod 100).
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = (5*m)^k for some positive m and some k > 1. - David A. Corneth, Apr 28 2022
Sum_{n>=1} 1/a(n) = -Sum_{k>=2} mu(k)*zeta(k)/5^k = 0.0756933503... - Amiram Eldar, Jul 02 2022
EXAMPLE
225 is a term since 225 = 15^2 is a power of a multiple of 5.
MATHEMATICA
Select[Range[32000], And[GCD @@ FactorInteger[#][[All, 2]] > 1, Divisible[#, 5]] &] (* Michael De Vlieger, Apr 30 2022 *)
PROG
(PARI) isok(k) = ispower(k) && !(k%5); \\ Michel Marcus, Apr 27 2022
(PARI) upto(n) = { my(res = List()); forstep(i = 5, sqrtint(n), 5, c = i; for(e = 2, logint(n, i), c*=i; listput(res, c) ) ); Set(res) } \\ David A. Corneth, Apr 27 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Marco Ripà, Apr 26 2022
STATUS
approved