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

Terms

a(0) =3a(1) =1a(2) =6a(3) =19a(4) =63a(5) =208a(6) =687a(7) =2269a(8) =7494a(9) =24751a(10) =81747a(11) =269992a(12) =891723a(13) =2945161a(14) =9727206a(15) =32126779a(16) =106107543a(17) =350449408a(18) =1157455767a(19) =3822816709a(20) =12625905894a(21) =41700534391a(22) =137727509067a(23) =454883061592