Generalized Fibonacci numbers D_{n,2}.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =8a(5) =16a(6) =34a(7) =72a(8) =154a(9) =336a(10) =738a(11) =1632a(12) =3640a(13) =8160a(14) =18384a(15) =41616a(16) =94560a(17) =215600a(18) =493122a(19) =1130976a(20) =2600388a(21) =5992560a(22) =13838306a(23) =32016576a(24) =74203112a(25) =172252080a(26) =400451282a(27) =932247360a(28) =2173035800a(29) =5071299968