Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Can we say (for sure) that "the function is increasing” to mean that the first derivative is positive?

Whenever $f'$ (the first derivative) is positive the function is increasing,but does that imply if a function is increasing the first derivative must be positive?

Please explain with an example.

share|improve this question
2  
No. First, increasing functions need not to be everywhere differentiable (for example $f\colon \mathbb R\to \mathbb R$ given by $f(x) = x$ for $x \ge 0$ and $f(x) = 2x$ for $x \le 0$), second: If $f$ is differentiable, you only need to have $f' \ge 0$, not $f' > 0$, for example: $g(x) = x^3$ has $g'(0) =0$. –  martini Oct 11 '11 at 16:29
3  
Increasing functions don't even have to be continuous! –  barrycarter Oct 11 '11 at 16:41

3 Answers 3

up vote 13 down vote accepted

No. Consider a dense countable subset $Q$ of $\mathbb R$ and a family $(a_q)_{q\in Q}$ of positive real numbers such that $\sum\limits_{q\in Q}a_q(1+|q|)$ converges. For every $x$, let $(x)^+=\max\{x,0\}$ denote the positive part of $x$.

Then, the function $f$ defined by $$ f(x)=\sum\limits_{q\in Q}a_q(x-q)^+ $$ is well defined for every real number $x$. The left and right derivatives of $f$ exist everywhere, with $$ f'_\ell(x)=\sum\limits_{q<x}a_q\qquad\text{and}\qquad f'_r(x)=\sum\limits_{q\leqslant x}a_q. $$ Thus, $f$ is strictly increasing and strictly convex, differentiable at every point not in $Q$, and not differentiable at every point in $Q$.

To prove the existence of $f'_\ell$ and $f'_r$ at every point, one can come back to the definitions of the left and right derivatives as the limits, if these exist, of $\pm(f(x\pm h)-f(x))/h$ when $h\to0^+$.

Or one can use directly the fact that each function $g_q$ defined by $g_q(x)=(x-q)^+$ has left and right derivatives $(g_q)'_\ell(x)=[x>q]$ and $(g_q)'_r(x)=[x\geqslant q]$.

share|improve this answer

Someone mentioned $f(x)=x^3$, for which the derivative at one point is $0$ but the function is everywhere increasing.

It can also happen that the derivative at one point is undefined and the function is everywhere increasing. For example, let $$ f(x) = \begin{cases} x & \text{if }x<0, \\ 2x & \text{if }x\ge 0. \end{cases} $$ The derivative is undefined at $x=0$. The function is everywhere increasing.

share|improve this answer

No. Consider function $f(x)=x^3$. It is increasing on $\mathbb R$ but $f'(0)=0$.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.