a(n)=2*Product[(1 + 4*Cos[k*Pi/n]^2)*(1 + 4*Sin[k*Pi/n]^2), {k, 1, Floor[(n - 1)/2]}] - Product[(1 + 4*Sin[k*Pi/n]^2), {k, 1, Floor[(n - 1)/2]}].

A152190

a(n)=2*Product[(1 + 4*Cos[k*Pi/n]^2)*(1 + 4*Sin[k*Pi/n]^2), {k, 1, Floor[(n - 1)/2]}] - Product[(1 + 4*Sin[k*Pi/n]^2), {k, 1, Floor[(n - 1)/2]}].

Terms

    a(0) =1a(1) =1a(2) =1a(3) =12a(4) =15a(5) =99a(6) =120a(7) =725a(8) =861a(9) =5092a(10) =5995a(11) =35223a(12) =41328a(13) =242265a(14) =283881a(15) =1662716a(16) =1947351a(17) =11402203a(18) =13351528a(19) =78166989a(20) =91523685a(21) =535804116a(22) =627341331a(23) =3672559727a(24) =4299936480a(25) =25172370289a(26) =29472399505a(27) =172534703340

External references