a(n) = 6^n - 3^n + 1.

Terms

a(0) =1a(1) =4a(2) =28a(3) =190a(4) =1216a(5) =7534a(6) =45928a(7) =277750a(8) =1673056a(9) =10058014a(10) =60407128a(11) =362619910a(12) =2176250896a(13) =13059099694a(14) =78359381128a(15) =470170635670