a(n) = 5*a(n-1) - 2*a(n-2) for n>1, with a(0) = 1, a(1) = 3.

Terms

a(0) =1a(1) =3a(2) =13a(3) =59a(4) =269a(5) =1227a(6) =5597a(7) =25531a(8) =116461a(9) =531243a(10) =2423293a(11) =11053979a(12) =50423309a(13) =230008587a(14) =1049196317a(15) =4785964411a(16) =21831429421a(17) =99585218283a(18) =454263232573