a(n) = 4*a(n-1) + a(n-2) - n for n > 1, with a(0) = 0, a(1) = 1.

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =22a(5) =89a(6) =372a(7) =1570a(8) =6644a(9) =28137a(10) =119182a(11) =504854a(12) =2138586a(13) =9059185a(14) =38375312a(15) =162560418a(16) =688616968a(17) =2917028273a(18) =12356730042a(19) =52343948422a(20) =221732523710a(21) =939274043241