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For example, $f(x)=\sin x$ changes concavity an infinite number of times, $f(x)=x^3-x$ has two regions of concavity (changing concavity once), and $f(x)=x$ changes $0$ times.

Is there a name for this property?

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    We could call it smart-Aleciness, maybe? ;P2011-10-06
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    How many times would you say $f(x)=x^4$ "changes concavity" ?2011-10-06
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    @Sasha $f(x)=x^4$ never changes concavity - it's second derivative never crosses from positive to negative.2011-10-06
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    Note that for functions that are differentiable everywhere, these points are *precisely* the local extremes of the derivative, so they are "local extremes of $f'(x)$". You could also call the points "flex points"; but there is no specific term that I am aware of.2011-10-06
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    As far as I know, $x^3$ only changes concavity once (at $x=0$). Where do you get "twice"? Points where a function changes concavity are commonly called "points of inflection", although there is some disagreement over the precise definition of this.2011-10-06
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    I think you're looking for the property which describes the place a t which a function changes concavity (point of inflection), and then if you want you can say the property is "has n points of inflection." Generally we wouldn't give a name to a property unless we study the property in depth. And AFAIK there are no significant theorems based on the number of times a function changes concavity.2011-10-06
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    @RobertIsrael Yes, you are correct. I had been thinking of $x^3-x$ and have since corrected my question.2011-10-10
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    @Bean That's sort of what I was getting at - I wanted to know if that property had any known significance to the properties and application of the function. If you put that as an answer, I will mark it as such.2011-10-10
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    How can $x^3-x$ change concavity twice? It is concave for large negative $x$, and convex for large positive $x$. Just from this, you know that it changes concavity an odd number of times.2011-10-10
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    @Alec: Still incorrect. $x^3-x$ changes concavity just once. Again, the concavity changes at the local extremes of the derivative. The derivative is $3x^2-1$, which is a quadratic, so it has a unique local extreme; hence $x^3-x$ changes concavity only once. Alternatively, $f''(x) = 6x$ is positive if $x\gt 0$, negative if $x\lt 0$, and changes concavity at $0$ and nowhere else.2011-10-10
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    I would call the property "number of times the function changes concavity"2011-10-15
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    @ArturoMagidin & TonyK You guys are right, it has two regions of different concavity, changing once, but the gist of the question remains the same.2011-10-17

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The points where a function changes concavity are called "flex points" or "points of inflection."

For functions with continuous derivatives, the changes in concavity occur exactly at the local extremes of derivative. Since a polynomial of degree $k$ has at most $k-1$ local extremes, it follows that a polynomial of degree $n$ has at most $n-2$ points of inflection. Moreover, by considering the sign of a polynomial as $x\to\infty$ and as $x\to-\infty$, it is easy to check that an odd degree polynomial will have an odd number of points of inflection, and an even degree polynomial will have an even number of points of inflection, except for polynomials of degree $1$.

In particular, your $f(x)=x^3-x$ cannot change concavity twice: it has at most (and in fact, exactly) one point of inflection.

Note that this simple analysis also means that polynomials of degree $3$ change concavity exactly once, and polynomials of degree $2$ never change concavity.

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I think you're looking for the property which describes the place at which a function changes concavity (point of inflection), and then, if you want, you can say the property is "$f$ has $n$ points of inflection." Generally we wouldn't give a name to a property unless we study the property in depth. And AFAIK there are no significant theorems based on the number of times a function changes concavity.

However, there are some problems being studied on real points of inflection on algebraic curves in a more general (tropical) setting. In the recent (2011) paper included below, the main theorem is:

A non-singular real algebraic curve in $\mathbb{R}P^2$ of degree $d \geq 3$ cannot have more than $d(d − 2)$ real inflection points.

Note that they still call the property "having $n$ inflection points"

http://www.math.jussieu.fr/~brugalle/articles/Inflection/Inflpoints.pdf