OFFSET
1,1
COMMENTS
Terms are multiples of 9, so that a(n) == 0 (mod 9) (since no perfect power divisible by 3 can have a 3-adic valuation below 2).
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
a(n) has the form (3*m)^k for some positive integer m := m(n) and some k > 1.
Sum_{n>=1} 1/a(n) = -Sum_{k>=2} mu(k)*zeta(k)/3^k = 0.2306128559... - Amiram Eldar, Jul 02 2022
EXAMPLE
36 is a term since 36 = (2*3)^2 is a power of a multiple of 3.
MAPLE
q:= n-> igcd(seq(i[2], i=ifactors(n)[2]))>1:
select(q, [9*i$i=1..2000])[]; # Alois P. Heinz, May 05 2022
MATHEMATICA
Select[9*Range[1400], GCD @@ FactorInteger[#][[All, 2]] > 1 &]
PROG
(PARI) isok(k) = ispower(k) && !(k % 3); \\ Michel Marcus, May 02 2022
CROSSREFS
Cf. A000244.
KEYWORD
nonn,easy
AUTHOR
Marco Ripà, May 02 2022
STATUS
approved