I did find it hidden in Fibonacci's Problem Book (M. Bicknell and V. E. Hoggatt, Jr, eds), pp. 47-48. Emeric On Sun, 15 Oct 2006, Emeric Deutsch wrote: > Dear Seqfan, > Is the following known? > > fibonacci(n)=Product(1 + 4[cos(j*Pi/n)]^2, j=1..ceil(n/2)-1) > > Thanks, > Emeric >