OFFSET
1,2
COMMENTS
This sequence is closed under multiplication.
The least term that is not a power of a prime (A000961) is a(14) = 5^2*7^2 = 1225.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Rough Number.
FORMULA
Sum_{n>=1} 1/a(n) = 4*zeta(2)*zeta(3)/(7*zeta(6)) = (4/7) * A082695 = 1.1106265353... .
MATHEMATICA
powQ[n_] := n == 1 || AllTrue[FactorInteger[n][[;; , 2]], # > 1 &]; Select[Flatten @ Table[6*n + {-1, 1}, {n, 0, 2000}], powQ]
PROG
(PARI) is(k) = gcd(k, 6) == 1 && ispowerful(k);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Amiram Eldar, Jul 02 2024
STATUS
approved