Solution
= Solution
Put
$$
F(s)=s+\frac{R(s)}c,
\qquad
R(s)=|x-y(s)|,
\qquad
n(s)=\frac{x-y(s)}{R(s)}.
$$
Because $dR/ds=-n\cdot v$, differentiation gives
$$
F'(s)=1-\frac{n\cdot v}{c}.
$$
A massive charge has the <subluminal speed> $|v|<c$, so $F'(s)>0$. Hence $F$ is <strictly increasing function>[strictly increasing], and $F(s)=t$ has at most one root. This is the <uniqueness of retarded time for a subluminal source>.