a(n) = 591*2^n - 273.

Terms

a(0) =318a(1) =909a(2) =2091a(3) =4455a(4) =9183a(5) =18639a(6) =37551a(7) =75375a(8) =151023a(9) =302319a(10) =604911a(11) =1210095a(12) =2420463a(13) =4841199a(14) =9682671a(15) =19365615a(16) =38731503a(17) =77463279a(18) =154926831a(19) =309853935a(20) =619708143a(21) =1239416559a(22) =2478833391a(23) =4957667055a(24) =9915334383a(25) =19830669039a(26) =39661338351