a(n) = Sum_{k=2..n} 6^k.

Terms

a(0) =0a(1) =36a(2) =252a(3) =1548a(4) =9324a(5) =55980a(6) =335916a(7) =2015532a(8) =12093228a(9) =72559404a(10) =435356460a(11) =2612138796a(12) =15672832812a(13) =94036996908a(14) =564221981484