a(0) = 1; for n > 0, a(n) = 3*2^(n-1) - 1.

Terms

a(0) =1a(1) =2a(2) =5a(3) =11a(4) =23a(5) =47a(6) =95a(7) =191a(8) =383a(9) =767a(10) =1535a(11) =3071a(12) =6143a(13) =12287a(14) =24575a(15) =49151a(16) =98303a(17) =196607a(18) =393215a(19) =786431a(20) =1572863a(21) =3145727a(22) =6291455a(23) =12582911a(24) =25165823a(25) =50331647a(26) =100663295a(27) =201326591a(28) =402653183a(29) =805306367