a(n) = (2^n - 1)^3 - 2.

Terms

a(0) =-2a(1) =-1a(2) =25a(3) =341a(4) =3373a(5) =29789a(6) =250045a(7) =2048381a(8) =16581373a(9) =133432829a(10) =1070599165a(11) =8577357821a(12) =68669157373a(13) =549554511869