4*n analog to Keith numbers.

Terms

a(0) =3a(1) =6a(2) =9a(3) =12a(4) =19a(5) =29a(6) =40a(7) =787a(8) =1679a(9) =2137a(10) =2508a(11) =2728a(12) =5016a(13) =7524a(14) =12773a(15) =36183a(16) =46116a(17) =192952a(18) =246916a(19) =681538a(20) =1316065a(21) =4826672a(22) =7571204a(23) =8709926a(24) =9716827a(25) =24922317