a(n) = 6^n - 1.

Terms

a(0) =0a(1) =5a(2) =35a(3) =215a(4) =1295a(5) =7775a(6) =46655a(7) =279935a(8) =1679615a(9) =10077695a(10) =60466175a(11) =362797055a(12) =2176782335a(13) =13060694015a(14) =78364164095a(15) =470184984575