eigenvalue-problem problem: $$ \Huge{} \begin{cases} \frac{y_{n+1} - 2 y_n + y_{n+1}}{h^2} = -\lambda y_n \\ -2 \frac{y_0 - y_1}{h^2} = -\lambda y_0 \\ y_N = y_{N-1} \end{cases} $$ compile: g++ main.cpp run: ./a.out 10