a(n) = (6^n - 5^n + 4^n - 3^n)/2.

Terms

a(0) =0a(1) =1a(2) =9a(3) =64a(4) =423a(5) =2716a(6) =17199a(7) =108004a(8) =673983a(9) =4183516a(10) =25845039a(11) =158993044a(12) =974443743a(13) =5952752716a(14) =36262150479a(15) =220363399684