a(n) = floor(2^n csc(1/n)).

Terms

a(0) =2a(1) =8a(2) =24a(3) =64a(4) =161a(5) =385a(6) =899a(7) =2053a(8) =4617a(9) =10257a(10) =22559a(11) =49208a(12) =106601a(13) =229571a(14) =491884a(15) =1049258a(16) =2229509a(17) =4721020a(18) =9966072a(19) =20980260a(20) =44056840a(21) =92306470a(22) =192998784a(23) =402769716a(24) =839084537a(25) =1745260723a(26) =3624707293a(27) =7517790835a(28) =15572342341a(29) =32218220725