# How to determine a shape is convex by giving polar form polynomial equation?

It is easy to determine concave, convex curve in xy coordinate. But I am placing a question that I only have a polar polynomial equation like r(ang) = a4*ang^4 + a3*ang^3 + .... + a0;

How I can tell whether it is convex shape by given a equation like this? ** Convert back to xy coordinate is NOT an allowable option.

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Polar equations that don't have trigonometric functions in them often look spiral-like... – J. M. Oct 28 '11 at 16:51
You are right, but assume I only care 1 complete turn (360 deg) and the discontinuous at 0deg/360deg can be ignored. Now how I can find whether the shape is convex? – Marco Oct 28 '11 at 17:21

## 1 Answer

In general for a simple smooth closed curve ${\bf R} = {\bf R}(t)$ traversed counterclockwise, let ${\bf v} = \frac{d{\bf R}}{dt}$ be the velocity vector and ${\bf a} = \frac{d{\bf v}}{dt}$ the acceleration. The curve is convex if $({\bf v} \times {\bf a})_3 \ge 0$ everywhere. For the polar curve $r = r(\theta)$ parametrized by $\theta$, if ${\bf u}_r$ and ${\bf u}_\theta$ are the unit vectors in the radial and counterclockwise directions, we have ${\bf v} = r' {\bf u}_r + r {\bf u}_\theta$ and ${\bf a} = (r'' - r) {\bf u}_r + 2 r' {\bf u}_\theta$, so ${\bf v} \times {\bf a} = (r^2 + 2 (r')^2 - r r'') {\bf u}_r \times {\bf u}_\theta$, and the condition is $r^2 + 2 (r')^2 - r r'' \ge 0$ for all $\theta$. Note that we can't just "ignore" a discontinuity in $r'$, which could correspond to a sharp corner that makes the curve non-convex.

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