PrA

PrA is a simple ad-hoc parameter to define the "non-circularity" or eccentricity of a 2D object. This quantity is simply:

${\begin{alignedat}{2}\mathrm {PRA} ={\frac {Pr}{A}}\end{alignedat}}$

Where $P$ is the object's perimeter, $A$ is its surface area, and $r$ is an effective size (radius), computed based on the corresponding circle of the same area:

${\begin{alignedat}{2}r={\sqrt {\frac {A}{\pi }}}\end{alignedat}}$

This definition of PrA is convenient, since it provides a simple measure of eccentricity. In particular, for a circle one expects:

${\begin{alignedat}{2}\mathrm {PRA} &={\frac {Pr}{A}}\\&={\frac {(2\pi r)(r)}{\pi r^{2}}}\\&=2\end{alignedat}}$

Since a circle has the minimal perimeter (for a given area), this is a limiting value of PrA:

${\begin{alignedat}{2}\mathrm {PRA} \geq 2\end{alignedat}}$

And thus any non-circular object will have a larger PrA. An infinitely eccentric object would have $\mathrm {PRA} \to \infty$.

Ellipse

If the object is an ellipse, with equation:

${\frac {x^{2}}{a^{2}}}+{\frac {y^{2}}{b^{2}}}=1$

Then the width is $2a$ and height $2b$ (we assume $a\geq b$), the foci are $(\pm c,0)$ for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle c = \sqrt{a^2-b^2}} . The eccentricity is:

$e={\frac {c}{a}}={\sqrt {1-{\frac {b^{2}}{a^{2}}}}}$

A circle has $e=0$, while increasingly squashed ellipses have values of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle e} closer and closer to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1} . The area of an ellipse is:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A = \pi a b}

The perimeter is not analytic but can be approximated very roughly by:

$P\approx \pi (a+b)$

Which yields:

${\begin{alignedat}{2}\mathrm {PRA} &={\frac {Pr}{A}}\\&={\frac {P\left({\sqrt {\frac {A}{\pi }}}\right)}{A}}\\&\approx {\frac {\pi (a+b)}{\pi ab}}\left({\sqrt {\frac {\pi ab}{\pi }}}\right)\\&\approx {\frac {(a+b)}{ab}}{\sqrt {ab}}\\&\approx {\frac {(a+b)}{\sqrt {ab}}}\\\end{alignedat}}$

One can establish a relationship between eccentricity and PrA by setting $b=1$ and considering Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a \in [1, \infty]} :

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} \mathrm{PRA} & \approx \frac{(a+1) }{ \sqrt{a} } \\ e & = \sqrt{1 - \frac{1}{a^2}} \end{alignat} }

So: