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

Terms

a(0) =1a(1) =0a(2) =24a(3) =198a(4) =1272a(5) =7746a(6) =46620a(7) =279894a(8) =1679568a(9) =10077642a(10) =60466116a(11) =362796990a(12) =2176782264a(13) =13060693938a(14) =78364164012a(15) =470184984486