a(1) = 2; a(n) = half of the a(n-1)-th even nontotient number.

Terms

a(0) =2a(1) =13a(2) =49a(3) =149a(4) =373a(5) =835a(6) =1727a(7) =3383a(8) =6341a(9) =11419a(10) =19966a(11) =34067a(12) =56967a(13) =93578a(14) =151313a(15) =241281a(16) =379934a(17) =591413a(18) =910879a(19) =1389289a(20) =2099905a(21) =3147743a(22) =4682362a(23) =6914234a(24) =10141072a(25) =14779023a(26) =21408820a(27) =30837491a(28) =44179913a(29) =62974570