a(n) = Product_{k=1..n} (1 + 4*sin(2*Pi*k/n)^2).

Terms

a(0) =0a(1) =1a(2) =1a(3) =16a(4) =25a(5) =121a(6) =256a(7) =841a(8) =2025a(9) =5776a(10) =14641a(11) =39601a(12) =102400a(13) =271441a(14) =707281a(15) =1860496a(16) =4862025a(17) =12752041a(18) =33362176a(19) =87403801a(20) =228765625a(21) =599074576a(22) =1568239201a(23) =4106118241a(24) =10749542400a(25) =28143753121a(26) =73680216481a(27) =192900153616