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This question already has an answer here:

I'd like to write the following equation:

Image of a mathematical equation.

I don't know which symbol is used for the P in the numerator. I have tried \mathcal{}, \mathfrak{}, and \mathsrc{}, but it wasn't any of these.

Could someone also recommend notation to describe the power density vector?

marked as duplicate by Werner, Phelype Oleinik, siracusa, Stefan Pinnow, Raaja May 31 at 4:00

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It is the notation for ‘Weierstraß' elliptic function’, called ‘Weierstraß P’, and obtained with the command \wp. It is in particular used for the parameterisation of elliptic cubic curves.

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    On StackExchange, it's best to always create a new question, instead of adding questions in comments. You may, however, find that this post answers your question. – Coby Viner May 30 at 18:02
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    You can increase it slighly with \mathlarger{\wp} (from the relsize package), or with something like, say, \text{\Large$\wp$}. – Bernard May 30 at 18:02
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    @Bernard - Unless one is writing in german, it's perfectly ok to write "Weierstrass" rather than "Weierstaß". :-) – Mico May 30 at 18:24
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    @Mico: Actually, I'd love to have the long s ligatures on my keyboard, but they seem to be unavailable… ;o) – Bernard May 30 at 18:38
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    Yes, it should be used for the Weierstraß p function, but it's abused by several authors. – egreg May 30 at 19:41
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As correctly indicated to you by the excellent users @barbara beeton both @Bernard the symbol to use is \wp. If I can help you, I have also inserted a code where I have used the clone of Times New Roman both in mathematical mode and in text mode.

enter image description here

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{newtxtext,newtxmath}
\begin{document}
\[D_H=\frac{\wp_{\max}}{P_{\text{rad}}/4\pi r^2}\]
\end{document}
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Additionally, the symbol ℘ for the Weierstrass elliptic function is in Unicode as U+2118. You can paste it directly into your source with unicode-math, of use \wp.

With legacy font packages, you can still define ℘ as \wp using newunicodechar (or inputenc).

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