"BHK" (reversible, identity, unlabeled) transform of 3,3,3,3,...

Terms

a(0) =3a(1) =6a(2) =21a(3) =87a(4) =363a(5) =1491a(6) =6051a(7) =24387a(8) =97923a(9) =392451a(10) =1571331a(11) =6288387a(12) =25159683a(13) =100651011a(14) =402628611a(15) =1610563587a(16) =6442352643a(17) =25769607171a(18) =103078821891a(19) =412316073987