TheoremBase

Inverse of a Continuous Strictly Increasing Function on an Interval: Continuity and Derivative

A continuous strictly increasing function on an interval maps it bijectively onto an interval; its inverse is strictly increasing and continuous, and is differentiable at f(x) with derivative 1/f'(x) wherever f has a nonzero derivative at x.

Statement

In the setting of The Real Line: Standing Notation and Background for Calculus on Intervals, let II be an interval containing two distinct points, let f:I→Rf:I\to\mathbb{R} be continuous on II and strictly increasing, that is, f(s)<f(t)f(s)<f(t) for all s,t∈Is,t\in I with s<ts<t, and let J=f(I)J=f(I) be the image of II under ff.

JJ is an interval containing two distinct points.

ff, read as a map I→JI\to J, is bijective.

The inverse g=f−1:J→Ig=f^{-1}:J\to I of this bijection is strictly increasing: g(y)<g(z)g(y)<g(z) for all y,z∈Jy,z\in J with y<zy<z.

The map gg, read as a map J→RJ\to\mathbb{R}, is continuous on JJ.

Let x∈Ix\in I. If ff is differentiable at xx and f′(x)≠0f'(x)\neq0, then gg, read as a map J→RJ\to\mathbb{R}, is differentiable at f(x)f(x), and g′(f(x))=1/f′(x)g'(f(x))=1/f'(x).

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