(n^2-n)*3^n.

Terms

a(0) =0a(1) =0a(2) =18a(3) =162a(4) =972a(5) =4860a(6) =21870a(7) =91854a(8) =367416a(9) =1417176a(10) =5314410a(11) =19486170a(12) =70150212a(13) =248714388a(14) =870500358a(15) =3013270470a(16) =10331213040a(17) =35126124336a(18) =118550669634a(19) =397493421714