a(n) = 3*a(n-1) - 2*(n-1), with a(0) = 1.

Terms

a(0) =1a(1) =3a(2) =7a(3) =17a(4) =45a(5) =127a(6) =371a(7) =1101a(8) =3289a(9) =9851a(10) =29535a(11) =88585a(12) =265733a(13) =797175a(14) =2391499a(15) =7174469a(16) =21523377a(17) =64570099a(18) =193710263a(19) =581130753a(20) =1743392221a(21) =5230176623a(22) =15690529827a(23) =47071589437a(24) =141214768265a(25) =423644304747