a(n) = n! + 3^n.

Terms

a(0) =2a(1) =4a(2) =11a(3) =33a(4) =105a(5) =363a(6) =1449a(7) =7227a(8) =46881a(9) =382563a(10) =3687849a(11) =40093947a(12) =479533041a(13) =6228615123a(14) =87183074169