a(n) = a(n-1) + 3*a(n-2) for n > 1, a(0) = a(1) = 1.

Terms

a(0) =1a(1) =1a(2) =4a(3) =7a(4) =19a(5) =40a(6) =97a(7) =217a(8) =508a(9) =1159a(10) =2683a(11) =6160a(12) =14209a(13) =32689a(14) =75316a(15) =173383a(16) =399331a(17) =919480a(18) =2117473a(19) =4875913a(20) =11228332a(21) =25856071a(22) =59541067a(23) =137109280a(24) =315732481a(25) =727060321a(26) =1674257764a(27) =3855438727a(28) =8878212019a(29) =20444528200