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A374289
5-rough powerful numbers: numbers k coprime to 6 such that if a prime p divides k then p^2 also divides k.
2
1, 25, 49, 121, 125, 169, 289, 343, 361, 529, 625, 841, 961, 1225, 1331, 1369, 1681, 1849, 2197, 2209, 2401, 2809, 3025, 3125, 3481, 3721, 4225, 4489, 4913, 5041, 5329, 5929, 6125, 6241, 6859, 6889, 7225, 7921, 8281, 8575, 9025, 9409, 10201, 10609, 11449, 11881, 12167
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.
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
Intersection of A007310 and A001694.
Intersection of A001651 and A062739.
A374290 is a subsequence.
Sequence in context: A241836 A110484 A110013 * A109861 A348754 A106564
KEYWORD
nonn,easy
AUTHOR
Amiram Eldar, Jul 02 2024
STATUS
approved