# Inflection points of bell shaped curves

The standard normal probability density function ${f(x) = \exp(-x^2/2)/\sqrt{2\pi}}$ has inflection points at ${x = \pm 1}$ where ${y=\exp(-1/2) /\sqrt{2\pi}\approx 0.24}$ which is about 60.5% of the maximum of this function. For this, as for other bell-shaped curves, the inflection points are also the points of steepest incline.

This is good to know for drawing an accurate sketch of this function, but in general, the Gaussian curve may be scaled differently, like ${A\exp(-B x^2)}$, and then the inflection points will be elsewhere. However, their relative height is invariant under scaling: it is always 60.5% of the maximum height of the curve. Since it is the height that we focus on, let us normalize various bell-shaped curves to have maximum ${1}$:

So, the Gaussian curve is inflected at the relative height of ${\exp(-1/2)\approx 0.605}$. For the Cauchy density ${1/(x^2+1)}$ the inflection is noticeably higher, at ${3/4 = 0.75}$ of the maximum:

Another popular bell shape, hyperbolic secant or simply ${2/(e^x+e^{-x})}$, is in between with inflection height ${1/\sqrt{2}\approx 0.707}$. It is slightly unexpected to see an algebraic number arising from this transcendental function.

Can we get inflection height below ${1/2}$? One candidate is ${1/(x^n+1)}$ with large even ${n}$, but this does not work: the relative height of inflection is $\frac{n+1}{2n} > \frac{1}{2}$. Shown here for ${n=10}$:

However, increasing the power of ${x}$ in the Gaussian curve works: for example, ${\exp(-x^4)}$ has inflection at relative height ${\exp(-3/4) \approx 0.47}$:

More generally, the relative height of inflection for ${\exp(-x^n)}$ is ${\exp(1/n - 1)}$ for even ${n}$. As ${n\to \infty}$, this approaches ${1/e\approx 0.37}$. Can we go lower?

Well, there are compactly supported bump functions which look bell-shaped, for example ${\exp(1/(x^2-1))}$ for ${|x|<1}$. Normalizing the height to ${1}$ makes it ${\exp(x^2/(x^2-1))}$. For this function inflection occurs at relative height about ${0.255}$.

Once again, we can replace ${2}$ by an arbitrary positive even integer ${n}$ and get relative inflection height down to ${\displaystyle\exp\left(-\left(1+\sqrt{5-4/n}\right)/2\right)}$. As ${n}$ increases, this height decreases to $e^{-\varphi}$ where ${\varphi}$ is the golden ratio. This is less than ${0.2}$ which is low enough for me today. The smallest ${n}$ for which the height is less than ${0.2}$ is ${n=54}$: it achieves inflection at ${y\approx 0.1999}$.

In the opposite direction, it is easy to produce bell-shaped curve with high inflection points: either ${1/(|x|^p + 1)}$ or ${\exp(-|x|^p)}$ will do, where ${p}$ is slightly larger than ${1}$. But these examples are only once differentiable, unlike the infinitely smooth examples above. Aside: as ${p\to 1}$, the latter function converges to the (rescaled) density of the Laplace distribution and the former to a non-integrable function.

As for the middle between two extremes… I did not find a reasonable bell-shaped curve that inflects at exactly half of its maximal height. An artificial example is ${\exp(-|x|^p)}$ with ${p=1/(1-\log 2) \approx 3.26}$ but this is ugly and only ${C^3}$ smooth.

## 3 thoughts on “Inflection points of bell shaped curves”

1. alexc84 says:

I’m getting lots of Yellow boxes with Red text say ‘Formula does not parse’; this affects 14 formulas in this post – similar problem in other posts.

1. alexc84 says:

all the formulas appear when the page loads but after about 1 second some them disappear behind these yellow error boxes

1. I saw a bunch of nonrendered formulas (and some rendered in completely wrong size/resolution) last night. But it works for me now.
The incident made me wonder about relying on the server-side rendering of formulas by WordPress dot com. Then again, if they break it, that will also destroy Terence Tao’s blog and people will notice enough to motivate them to fix it.

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