a(n) = 5^n-4^n-1.

Terms

a(0) =0a(1) =8a(2) =60a(3) =368a(4) =2100a(5) =11528a(6) =61740a(7) =325088a(8) =1690980a(9) =8717048a(10) =44633820a(11) =227363408a(12) =1153594260a(13) =5835080168a(14) =29443836300a(15) =148292923328a(16) =745759583940