%N Positive numbers divisible by 3^3 or by the square of some other prime.
%C Or numbers n>=4 having a divisor k^2>=4 such that n and n/k^2 equal modulo 3.
%C All positive multiples of 4 are in the sequence.
%C 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).
%C Or complement to the sequence: S, 3*S and 9*S, where S denotes the sequence of the squarefree numbers not divisible by 3.
%F 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.
%o (PARI) isok(n) = (((v=valuation(n, 3)) >= 3) || (((m = n/3^v) > 1) && (vecmax((factor(m))[,2]) >=2))); \\ _Michel Marcus_, Jun 12 2016
%Y Cf. A046790, A046791, A007913.
%A _Vladimir Shevelev_, Jun 11 2016