Past exam of the mathematics course of the University of Cambridge / 2008 / ii / Paper 2 / 11F / Solution

Let $1$ be the constant function and define $h=L1\in C([0,1])$. Positivity gives $h\ge 0$. Fix any $x$ and any continuous $f$. The function $g=f-f(x)1$ vanishes at $x$, so the zero-preserving assumption gives $Lg(x)=0$. By linearity this is $Lf(x)-f(x)L1(x)=0$. Since $x$ was arbitrary,This proves the zero-preserving linear operator on continuous functions representation. Zero preservation and linearity determine the multiplication operator; positivity is needed only for the multiplier's sign. The three numbered conditions in the source are hypotheses of this one request, rather than three separate subquestions.