a(n) = 4*a(n-1)+a(n-2), n>2; a(0)=1, a(1)=3, a(2)=12.

Terms

a(0) =1a(1) =3a(2) =12a(3) =51a(4) =216a(5) =915a(6) =3876a(7) =16419a(8) =69552a(9) =294627a(10) =1248060a(11) =5286867a(12) =22395528a(13) =94868979a(14) =401871444a(15) =1702354755a(16) =7211290464a(17) =30547516611a(18) =129401356908a(19) =548152944243