[Tex/LaTex] To write dot product \cdot is not working

equationsgreekmath-mode

I am trying to write the following equation:

enter image description here

But when I write this in the equation:

\documentclass[applsci,article,submit,moreauthors,pdftex,10pt,a4paper]{Definitions/mdpi} 
\begin{document}
\begin{equation} \label{eq:1}
\nabla \cdot \mathbf{\sigma+f^{b}}=0
\end{equation}
\end{document}

It gives me this:

enter image description here

This is not working for a specific class of file, a journal prescribed. I have used this similar expression elsewhere and it has worked.

Best Answer

I have two suggestions:

  • Load the amsmath package and rewrite the equation as 

    \nabla \cdot \boldsymbol{\sigma} + \mathbf{f}^{\mathbf{b}}=0

  • Load the bm ("bold math") package and rewrite the equation as 

    \nabla \cdot \bm{\sigma} + \bm{f^b}=0

The former method will generate upright-bold letters "f" and "b"; the second will generate italic-bold letters "f" and "b". Your choice.

enter image description here

Note that the + symbol should (almost certainly) not be rendered in bold. For sure, \mathbf{\sigma} is not good; write either \boldsymbol{\sigma} or \bm{\sigma}.

\documentclass[applsci,article,submit,moreauthors,
               pdftex,10pt,a4paper]{mdpi} % see https://www.mdpi.com/authors/latex
\usepackage{amsmath} % for "\boldsymbol" macro
\usepackage{bm}      % for "\bm" macro

\makeatletter  % just to make the code minimally compilable
\let\@Title\relax
\let\@Author\relax
\let\@address\relax
\makeatother

\begin{document}

\begin{equation} \label{eq:boldsymb}
\nabla \cdot \boldsymbol{\sigma} + \mathbf{f}^{\mathbf{b}}=0
\end{equation}

\begin{equation} \label{eq:bm}
\nabla \cdot \bm{\sigma} + \bm{f^b}=0
\end{equation}

\end{document}
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