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

Terms

a(0) =0a(1) =1a(2) =3a(3) =12a(4) =45a(5) =171a(6) =648a(7) =2457a(8) =9315a(9) =35316a(10) =133893a(11) =507627a(12) =1924560a(13) =7296561a(14) =27663363a(15) =104879772a(16) =397629405a(17) =1507527531a(18) =5715470808a(19) =21668995017a(20) =82153397475a(21) =311467177476