I have tried to find the derivation of the formula for the number of tilings of an $m \times n$ board with $2 \times 1$ tiles which is the following.
$$\prod_{k=1}^{m}\prod_{l=1}^{n} \left(4\cos^2{\frac{k\pi}{m + 1} + 4\cos^2{\frac{l\pi}{n + 1}}}\right)^{\frac{1}{2}}$$
Could anyone show me how to derive it or where I could find the derivation for this incredible formula?