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Numbers n such that phi(n) divides rad(n).
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%I #10 May 13 2013 01:49:40

%S 1,2,4,6,18

%N Numbers n such that phi(n) divides rad(n).

%C Terms are all of the form n = 2^a * 3^b with a <= 2 (else 4 | rad(n)) and b <= 2 (else 9 | rad(n)), hence the sequence is finite. [_Charles R Greathouse IV_, Jun 29 2011]

%t Rad[n_] := Times @@ Transpose[FactorInteger[n]][[1]]; Select[Range[10^6], Mod[Rad[#], EulerPhi[#]] == 0 &]

%o (PARI) rad(n)=n=factor(n)[,1];prod(i=1,#n,n[i])

%o for(n=1,2^2*3^2,if(rad(n)%eulerphi(n)==0,print1(n", "))) \\ _Charles R Greathouse IV_, Jun 29 2011

%Y Cf. A000010, A007947.

%K nonn,fini,full

%O 1,2

%A _José María Grau Ribas_, Jun 28 2011