Past exam of the mathematics course of the University of Cambridge / 2019 / ii / Paper 1 / 27K / b / iii / Solution

First, $f(0)=1$. For every $r\ge 0$,It cannot vanish: if $f(r_0)=0$ for some $r_0>0$, thenwould imply $f(r_0/n)=0$ for every positive integer $n$, contradicting continuity at zero. Thus $f(r)>0$. Since every characteristic function has modulus at most one,

Now set $h(r)=-\log f(r)$. It is continuous, nonnegative, and additive:The classification of a continuous additive function on the nonnegative real numbers gives $h(r)=\alpha r$, where $\alpha =h(1)\ge 0$. Consequently