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

Terms

a(0) =1a(1) =3a(2) =8a(3) =24a(4) =75a(5) =233a(6) =714a(7) =2166a(8) =6533a(9) =19647a(10) =59004a(11) =177092a(12) =531375a(13) =1594245a(14) =4782878a(15) =14348802a(16) =43046601a(17) =129140027a(18) =387420336a(19) =1162261296a(20) =3486784211a(21) =10460352993a(22) =31381059378a(23) =94143178574a(24) =282429536205a(25) =847288609143