a(n) = 6^n - n^2.

Terms

a(0) =1a(1) =5a(2) =32a(3) =207a(4) =1280a(5) =7751a(6) =46620a(7) =279887a(8) =1679552a(9) =10077615a(10) =60466076a(11) =362796935a(12) =2176782192a(13) =13060693847a(14) =78364163900a(15) =470184984351