OFFSET
1,1
COMMENTS
Or numbers n>=4 having a divisor k^2>=4 such that n and n/k^2 equal modulo 3.
All positive multiples of 4 are in the sequence.
Or numbers n such that there is a smaller positive number j == n (mod 3) such that sqrt(j*n) is integer. The smallest such j corresponds to the greatest k; or, the same, j = 3*A007913(n/3), if n is divisible by 3 and otherwise j=A007913(n).
Or complement to the sequence: S, 3*S and 9*S, where S denotes the sequence of the squarefree numbers not divisible by 3.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
Let A(x) be the number of a(n)<=x. Then A(x)~(1 - 6.5/Pi^2)*x = 0.34141230...*x as x goes to infinity.
MATHEMATICA
Select[Range[200], (e = IntegerExponent[#, 3]) > 2 || ! SquareFreeQ[#/3^e] &] (* Amiram Eldar, Feb 25 2021 *)
PROG
(PARI) isok(n) = (((v=valuation(n, 3)) >= 3) || (((m = n/3^v) > 1) && (vecmax((factor(m))[, 2]) >=2))); \\ Michel Marcus, Jun 12 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Shevelev, Jun 11 2016
STATUS
approved