Let M= [0, -α; α, 0], where α is a non-zero real number an N=Σ_k=1^49 M^2 k. If (I-M^2) N=-2 I, then the positive integral value of α is \ \ \ \.

Let $\displaystyle M=\left[\begin{array}{cc}0 & -\alpha \\ \alpha & 0\end{array}\right]$, where $\displaystyle \alpha$ is a non-zero real number an $\displaystyle N=\sum_{k=1}^{49} M^{2 k}$. If $\displaystyle \left(I-M^2\right) N=-2 I$, then the positive integral value of $\displaystyle \alpha$ is \_\_\_\_.