Prove that the height of the cylinder of maximum volume inscribed in a sphere of radius \( R \) is \( \frac{2R}{\sqrt{3}} \).
Consider the domain \( D = \{ (x, y) \in \mathbb{R}^2 : x \leq y \} \) and the function \( h : D \to \mathbb{R} \) defined by \[ h((x, y)) = (x - 2)^4 + (y - 1)^4, (x, y) \in D. \] Then the minimum value of \( h \) on \( D \) equals