a(n) = 6^n - n^3.

Terms

a(0) =1a(1) =5a(2) =28a(3) =189a(4) =1232a(5) =7651a(6) =46440a(7) =279593a(8) =1679104a(9) =10076967a(10) =60465176a(11) =362795725a(12) =2176780608a(13) =13060691819a(14) =78364161352a(15) =470184981201