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

Terms

a(0) =1a(1) =6a(2) =17a(3) =40a(4) =87a(5) =182a(6) =373a(7) =756a(8) =1523a(9) =3058a(10) =6129a(11) =12272a(12) =24559a(13) =49134a(14) =98285a(15) =196588a(16) =393195a(17) =786410a(18) =1572841a(19) =3145704a(20) =6291431a(21) =12582886a(22) =25165797a(23) =50331620a(24) =100663267a(25) =201326562a(26) =402653153a(27) =805306336a(28) =1610612703a(29) =3221225438