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

Terms

a(0) =1a(1) =4a(2) =15a(3) =50a(4) =157a(5) =480a(6) =1451a(7) =4366a(8) =13113a(9) =39356a(10) =118087a(11) =354282a(12) =1062869a(13) =3188632a(14) =9565923a(15) =28697798a(16) =86093425a(17) =258280308a(18) =774840959a(19) =2324522914a(20) =6973568781a(21) =20920706384a(22) =62762119195a(23) =188286357630a(24) =564859072937