What is this symbol?

Question

In this article one can find the following symbol:

I have tried to Detexify it to no avail. What is this symbol/font?

It appears many times in the following excerpt:

Answer 1

The letter is an uppercase “N” in a Fraktur style alphabet.

Unless you find a Fraktur font in which the letter has that precise shape, you can add

\usepackage{amssymb}

\newcommand{\cobordism}{\mathfrak{N}}

to your preamble (also \usepackage{amsfonts} would suffice, but usually amssymb is loaded, because it gives access to a wealth of math symbols). The “exact” shape is not important, so long as the symbol is a recognizable “Fraktur N”.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}

\usepackage{amsmath}
\usepackage{amssymb}

\newcommand{\cobordism}{\mathfrak{N}}

\begin{document}

\[
\cobordism L_n \cong \pi_n(M\lambda); L_n \cong (M\tilde{\lambda}).
\]
Dans ce travail, nous rappelons le calcul de $\cobordism L_*$ et
nous calculons l'image de $\cobordism L_*$ dans le cobordisme
non-orienté $\cobordism_*$ de Thom.

\end{document}