Darboux intermediate-value property of derivatives
For f differentiable throughout an open real interval, its derivative takes every value strictly between f′(a) and f′(b) at some c∈(a,b), whenever a<b are points of that interval.
Key knowledge
For f differentiable throughout an open real interval, its derivative takes every value strictly between f′(a) and f′(b) at some c∈(a,b), whenever a<b are points of that interval. A derivative need not be continuous, but it cannot skip an intermediate derivative value.
Conditions and scope
- Both a and b are interior points where the derivative exists; no one-sided endpoint convention is needed here.
- The argument applies compact-domain extrema to f(x)−yx or yx−f(x), then uses the interior extremum derivative condition.
Sources · 1
References
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Evidence and review
Theorem 4.2.11 and both sign cases of the maximum/Fermat proof; scope restricted to interior interval points.
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