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

Terms

a(0) =0a(1) =3a(2) =9a(3) =36a(4) =135a(5) =513a(6) =1944a(7) =7371a(8) =27945a(9) =105948a(10) =401679a(11) =1522881a(12) =5773680a(13) =21889683a(14) =82990089a(15) =314639316a(16) =1192888215a(17) =4522582593a(18) =17146412424a(19) =65006985051a(20) =246460192425a(21) =934401532428