a(n) = Sum_{k=1..n} k^2 * 3^(n-k).

Terms

a(0) =0a(1) =1a(2) =7a(3) =30a(4) =106a(5) =343a(6) =1065a(7) =3244a(8) =9796a(9) =29469a(10) =88507a(11) =265642a(12) =797070a(13) =2391379a(14) =7174333a(15) =21523224a(16) =64569928a(17) =193710073a(18) =581130543a(19) =1743391990a(20) =5230176370a(21) =15690529551a(22) =47071589137a(23) =141214767940a(24) =423644304396