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

Terms

a(0) =4a(1) =32a(2) =145a(3) =514a(4) =1605a(5) =4618a(6) =12565a(7) =32810a(8) =83029a(9) =204970a(10) =495957a(11) =1180330a(12) =2770261a(13) =6425259a(14) =14751062a(15) =33565356a(16) =75781465a(17) =169913009a(18) =378623329a(19) =839035584a(20) =1850037628a(21) =4060785394a(22) =8876545497a(23) =19330149277a(24) =41948632852a(25) =90742369765a(26) =195711818579a(27) =420951537104