a(n) = (1/2)*n*(n-1)*3^(n-1).

Terms

a(0) =0a(1) =0a(2) =3a(3) =27a(4) =162a(5) =810a(6) =3645a(7) =15309a(8) =61236a(9) =236196a(10) =885735a(11) =3247695a(12) =11691702a(13) =41452398a(14) =145083393a(15) =502211745a(16) =1721868840a(17) =5854354056a(18) =19758444939a(19) =66248903619a(20) =220829678730a(21) =732224724210