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

Terms

a(0) =2a(1) =2a(2) =1a(3) =0a(4) =1a(5) =8a(6) =29a(7) =80a(8) =193a(9) =432a(10) =925a(11) =1928a(12) =3953a(13) =8024a(14) =16189a(15) =32544a(16) =65281a(17) =130784a(18) =261821a(19) =523928a(20) =1048177a(21) =2096712a(22) =4193821a(23) =8388080a(24) =16776641a(25) =33553808a(26) =67108189a(27) =134217000a(28) =268434673a(29) =536870072