a(n) = 6*2^(n+1) - 5*(n+1) - 4.

Terms

a(0) =3a(1) =10a(2) =29a(3) =72a(4) =163a(5) =350a(6) =729a(7) =1492a(8) =3023a(9) =6090a(10) =12229a(11) =24512a(12) =49083a(13) =98230a(14) =196529a(15) =393132a(16) =786343a(17) =1572770a(18) =3145629a(19) =6291352a(20) =12582803a(21) =25165710a(22) =50331529a(23) =100663172a(24) =201326463a(25) =402653050a(26) =805306229a(27) =1610612592a(28) =3221225323