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

Terms

a(0) =1a(1) =4a(2) =10a(3) =22a(4) =46a(5) =94a(6) =190a(7) =382a(8) =766a(9) =1534a(10) =3070a(11) =6142a(12) =12286a(13) =24574a(14) =49150a(15) =98302a(16) =196606a(17) =393214a(18) =786430a(19) =1572862a(20) =3145726a(21) =6291454a(22) =12582910a(23) =25165822a(24) =50331646a(25) =100663294a(26) =201326590a(27) =402653182a(28) =805306366a(29) =1610612734