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What is the letter in red box. How to write it in LATEX? Thank all! et

marked as duplicate by Raaja, Stefan Pinnow, Sverre, Sebastiano, Phelype Oleinik Jul 31 at 11:30

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(too long for a comment, hence posted as an answer)

The letter is a calligraphic uppercase "J", likely provided by the Computer Modern (math) font family.

enter image description here

Various math fonts can render this letter quite differently. I suggest you study pp. 6-8 of the user guide of the mathalfa package for several dozen examples of \mathcal{J} for various math fonts.

The resolution of the screenshot you posted is not very high. Thus, it will invariably lose detail when compared with the screenshot above, possibly giving rise to the impression that the shapes of the two letters aren't fully identical.

\documentclass[border=1pt]{standalone}
\begin{document}
$\displaystyle \mathcal{J}^{-1}$
\end{document}
  • Sorry for the basic question, but why use \displaystyle? What is the purpose of this? – AndréC Jul 31 at 7:52
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    @AndréC - The \displaystyle directive is there to maximize compatibility with the displayed equation in the OP's screenshot. In the screenshot I posted, the main effect of providing a \displaystyle directive is to raise the superscript term (-1) by a point or so, relative to what would be the case if a \displaystyle directive had not been provided. – Mico Jul 31 at 8:06
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    thanks you very much :) – AndréC Jul 31 at 8:11

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