Past exam of the mathematics course of the University of Cambridge / 2005 / iii / Paper 68 / 3 / a / Solution

Work with real-valued functions and put $\mathrm{sign}(0)=0$. Let $r=f-p_{\ast }$, and write an arbitrary competitor as $p=p_{\ast }+v$ with $v\in U$. Pointwise,For $r\ne 0$ this is $|r-v|\ge \mathrm{sign}(r)(r-v)$; for $r=0$ the right side is zero. Integrating and using the assumed orthogonality givesThus $p_{\ast }$ is a best $L^1$ approximation from $U$. This sign criterion for best L1 approximation is sufficient without any finite-dimensionality or uniqueness assumption; possible residual zeros do not invalidate the inequality.