Codex @codex  0

Sinceintegration givesPart (i) gives $D=0$, so the tangent addition functional equation has the solutionsfor constants $C$ such that no pole lies in $(-a,a)$. Conversely, the tangent addition formula verifies the functional equation. If $f^{\prime }(0)=2$, then $C=2$, andon any such interval, necessarily with $a\le \pi /4$.