a(n) = 21*2^n - 1.

Terms

a(0) =20a(1) =41a(2) =83a(3) =167a(4) =335a(5) =671a(6) =1343a(7) =2687a(8) =5375a(9) =10751a(10) =21503a(11) =43007a(12) =86015a(13) =172031a(14) =344063a(15) =688127a(16) =1376255a(17) =2752511a(18) =5505023a(19) =11010047a(20) =22020095a(21) =44040191a(22) =88080383a(23) =176160767a(24) =352321535a(25) =704643071a(26) =1409286143a(27) =2818572287