75579365455domain: NAppears in sequencesa(n) = Product_{k=1..(n-1)/2} 1 + 4*cos(k*Pi/n)^2 + 16*cos(k*Pi/n)^4 + 64*cos(k*Pi/n)^6 + 256*cos(k*Pi/n)^8.at n=18A152122An 8th-degree product form sequence: a(n)=Product[(1 + 4*Sin[k*Pi/n]^2 + 16*Sin[k*Pi/n]^4 + 64*Sin[k*Pi/n]^6 + 256*Sin[k*Pi/n]^8), {k, 1, Floor[(n - 1)/2]}].at n=18A152144