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

Terms

a(0) =39a(1) =55a(2) =111a(3) =183a(4) =219a(5) =459a(6) =471a(7) =579a(8) =831a(9) =867a(10) =939a(11) =1191a(12) =1263a(13) =1371a(14) =1623a(15) =1839a(16) =1983a(17) =2019a(18) =2199a(19) =2271a(20) =2631a(21) =2991a(22) =3279a(23) =3459a(24) =3603a(25) =3639a(26) =3711a(27) =3963a(28) =4143a(29) =4359