a(n) = 10*2^(n-1) - 9.

Terms

a(0) =1a(1) =11a(2) =31a(3) =71a(4) =151a(5) =311a(6) =631a(7) =1271a(8) =2551a(9) =5111a(10) =10231a(11) =20471a(12) =40951a(13) =81911a(14) =163831a(15) =327671a(16) =655351a(17) =1310711a(18) =2621431a(19) =5242871a(20) =10485751a(21) =20971511a(22) =41943031a(23) =83886071a(24) =167772151a(25) =335544311a(26) =671088631a(27) =1342177271a(28) =2684354551a(29) =5368709111