a(n+1) = 4*a(n) - n.

Terms

a(0) =1a(1) =3a(2) =10a(3) =37a(4) =144a(5) =571a(6) =2278a(7) =9105a(8) =36412a(9) =145639a(10) =582546a(11) =2330173a(12) =9320680a(13) =37282707a(14) =149130814a(15) =596523241a(16) =2386092948a(17) =9544371775a(18) =38177487082a(19) =152709948309a(20) =610839793216