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

Terms

a(0) =1a(1) =1a(2) =0a(3) =-1a(4) =0a(5) =7a(6) =28a(7) =79a(8) =192a(9) =431a(10) =924a(11) =1927a(12) =3952a(13) =8023a(14) =16188a(15) =32543a(16) =65280a(17) =130783a(18) =261820a(19) =523927a(20) =1048176a(21) =2096711a(22) =4193820a(23) =8388079a(24) =16776640a(25) =33553807a(26) =67108188a(27) =134216999a(28) =268434672a(29) =536870071