a(n) = (2n + 1)*3^n.

Terms

a(0) =1a(1) =9a(2) =45a(3) =189a(4) =729a(5) =2673a(6) =9477a(7) =32805a(8) =111537a(9) =373977a(10) =1240029a(11) =4074381a(12) =13286025a(13) =43046721a(14) =138706101a(15) =444816117a(16) =1420541793a(17) =4519905705a(18) =14334558093a(19) =45328197213a(20) =142958160441a(21) =449795187729