A063480 - OEIS

%I #10 Dec 15 2017 17:35:22

%S 39,55,111,183,219,459,471,579,831,867,939,1191,1263,1371,1623,1839,

%T 1983,2019,2199,2271,2631,2991,3279,3459,3603,3639,3711,3963,4143,

%U 4359,4863,4947,4971,5259,5619,5799,5979,6051,6411,7023,7107,7419,7671,7779

%N C(n+3)=2*C(n), where C(n) is Cototient(n) := n - phi(n) (A051953).

%H Harry J. Smith, <a href="/A063480/b063480.txt">Table of n, a(n) for n=1,...,1000</a>

%e C(39) = 15, C(39+3) = 2*15.

%o (PARI) C(n)=n-eulerphi(n); j=[]; for(n=1,20000, if(C(n+3)==2*C(n),j=concat(j,n))); j

%o (PARI) { n=0; c1=c2=c3=1; for (m=1, 10^9, c=c1; c1=c2; c2=c3; c3=m-eulerphi(m); if (c3==2*c, write("b063480.txt", n++, " ", m - 3); if (n==1000, break)) ) } \\ _Harry J. Smith_, Aug 23 2009

%Y Cf. A059153.

%K nonn

%O 1,1

%A _Jason Earls_, Jul 28 2001