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

Terms

a(0) =0a(1) =2a(2) =7a(3) =19a(4) =47a(5) =111a(6) =255a(7) =575a(8) =1279a(9) =2815a(10) =6143a(11) =13311a(12) =28671a(13) =61439a(14) =131071a(15) =278527a(16) =589823a(17) =1245183a(18) =2621439a(19) =5505023a(20) =11534335a(21) =24117247a(22) =50331647a(23) =104857599a(24) =218103807a(25) =452984831a(26) =939524095