a(n) = floor(2^n/(-1 + cot(1/n))).

Terms

a(0) =-6a(1) =4a(2) =4a(3) =5a(4) =8a(5) =12a(6) =21a(7) =36a(8) =64a(9) =114a(10) =205a(11) =373a(12) =684a(13) =1262a(14) =2344a(15) =4375a(16) =8202a(17) =15437a(18) =29155a(19) =55236a(20) =104940a(21) =199872a(22) =381551a(23) =729884a(24) =1398878a(25) =2685731a(26) =5164672a(27) =9946439a(28) =19181836a(29) =37039772