a(n) = 4*6^n-1.

Terms

a(0) =3a(1) =23a(2) =143a(3) =863a(4) =5183a(5) =31103a(6) =186623a(7) =1119743a(8) =6718463a(9) =40310783a(10) =241864703a(11) =1451188223a(12) =8707129343a(13) =52242776063a(14) =313456656383