Here is the Latex:

 % !TeX spellcheck = en_US
\documentclass[11 pt]{extarticle}
\usepackage{fouriernc} % Use this 
\usepackage{  amsthm, amssymb,latexsym,amsfonts}


\mathbb{Var}\left(X_n \right) = \mathbb{E} \left( \mathbb{Var}\left(X_n| X_{n-1} \right) \right) +
\mathbb{Var}\left( \mathbb{E}\left(X_n| X_{n-1} \right)  \right) . 

And here what I get: enter image description here

Any idea? Thanks.

  • 5
    There is no lower case letters for mathbb. – Sigur Mar 13 '20 at 11:42
  • 1
    See e.g. here for getting lower-case double-stroke letters. – user194703 Mar 13 '20 at 11:43
  • 1
    From the link above the solution tex.stackexchange.com/a/400371 also works with XeLaTeX (and not just LuaLaTeX as is sort of implied). MWE: \documentclass[11 pt]{extarticle} \usepackage[reqno,fleqn]{amsmath} \usepackage{fontspec} \usepackage{unicode-math} \setmathfont{Latin Modern Math} \begin{document} $\mathbb{Var}$ \end{document} – Marijn Mar 13 '20 at 11:48
  • Marijn, This does not work for me. – Rustam Mamin Mar 13 '20 at 12:24
  • 2
    Best to update everything, packages are interdependent so the most recent version of one package may depend on a recent version of another package. – Marijn Mar 13 '20 at 20:55

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