a(n) = [2^n csc(1/n)] - [2^n cot(1/n)], where [ ] = floor.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =5a(6) =10a(7) =16a(8) =28a(9) =52a(10) =94a(11) =170a(12) =316a(13) =586a(14) =1093a(15) =2048a(16) =3856a(17) =7284a(18) =13800a(19) =26220a(20) =49942a(21) =95341a(22) =182390a(23) =349576a(24) =671178a(25) =1290714a(26) =2485798a(27) =4793999a(28) =9257313a(29) =17897354