Bourlets Theorem for the product of differential operators, an application of the operator method and a proof for $sum_{n=1}^{infty}frac{1}{n^2}=frac{pi^2}{6}$, that Euler missed, derived from difference equations

We give another proof for [ sum_{n=1}^{infty}frac{1}{n^2}=frac{pi^2}{6} ] that basically follows from the theory of difference equations.