a(n) = 8^n - 2^n.

Terms

a(0) =0a(1) =6a(2) =60a(3) =504a(4) =4080a(5) =32736a(6) =262080a(7) =2097024a(8) =16776960a(9) =134217216a(10) =1073740800a(11) =8589932544a(12) =68719472640a(13) =549755805696