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

Terms

a(0) =1a(1) =4a(2) =19a(3) =82a(4) =325a(5) =1216a(6) =4375a(7) =15310a(8) =52489a(9) =177148a(10) =590491a(11) =1948618a(12) =6377293a(13) =20726200a(14) =66961567a(15) =215233606a(16) =688747537a(17) =2195382772a(18) =6973568803a(19) =22082967874a(20) =69735688021a(21) =219667417264a(22) =690383311399