Let $I$ be an interval in the sense of \ref{def:interval-real-line-c54-2026a}, let $f:I\to\mathbb{R}$, let $c\in I$ be an \ref{def:interior-point-interval-c54-2026a}, and assume that $f$ is differentiable at $c$ in the sense of \ref{def:derivative-interior-point-c54-2026b}. If $f$ has a local extremum at $c$ in the sense of \ref{def:local-extremum-at-point-1d-2026a}, then