https://medium.com/@hosamedwee/bellman-equation-1-understanding-the-recursive-nature-of-the-bellman-equation-in-mathematics-cd71bf14be1a
https://medium.com/@hosamedwee/bellman-equation-2-the-convergence-property-of-the-bellman-equation-5eef03df7795
Monotone Convergence Theorem