a(1) = 23, a(2) = 71, a(n) = 3*a(n-1) - 2*a(n-2) for n>2.

Terms

a(0) =23a(1) =71a(2) =167a(3) =359a(4) =743a(5) =1511a(6) =3047a(7) =6119a(8) =12263a(9) =24551a(10) =49127a(11) =98279a(12) =196583a(13) =393191a(14) =786407a(15) =1572839a(16) =3145703a(17) =6291431a(18) =12582887a(19) =25165799a(20) =50331623a(21) =100663271a(22) =201326567a(23) =402653159a(24) =805306343a(25) =1610612711a(26) =3221225447a(27) =6442450919a(28) =12884901863a(29) =25769803751