a(n) = 4*a(n-1) + 3*a(n-2), with a(0)=0, a(1)=3.

Terms

a(0) =0a(1) =3a(2) =12a(3) =57a(4) =264a(5) =1227a(6) =5700a(7) =26481a(8) =123024a(9) =571539a(10) =2655228a(11) =12335529a(12) =57307800a(13) =266237787a(14) =1236874548a(15) =5746211553a(16) =26695469856a(17) =124020514083a(18) =576168465900