amsmath and amsfonts incompatibility issues

Question

In my code, I was using the amsmath package and some others and the command \widehat was working just fine, but when I added the package amsfonts, the output of \widehat started producing an error: the variable inside the command is duplicated in the output. For example, \widehat{y}_{t_q} produces:

If I remove the amsfonts package from the code, then the problem disappears. I would like to know how can I use both packages and avoid this error in the \widehat command. Thank you in advance for your attention and advice!

Erica

A short version of my code follows:

\documentclass[11pt]{article}

\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{indentfirst}
\usepackage{float}
\usepackage{array}
\usepackage{breqn}
\usepackage{natbib}
\usepackage{hyperref}
\usepackage{amsfonts}

\bibpunct{(}{)}{,}{a}{,}{,}
\newcommand{\BibTeX}{{\sc Bib}\TeX}
\renewcommand{\baselinestretch}{1.2}

\newenvironment{proof}[1][Proof]{\noindent\textbf{#1.} }{\ \rule{0.5em}{0.5em}}
\usepackage{a4wide}

\begin{document}

\begin{equation} \label{70}
\left( \begin{array}{c}
              v_{t_m} \\
              u_{t_m} 
   \end{array} \right) \sim N \left( 0, \left[ \begin{array}{cc}
              \Sigma_{vv} & 0 \\
              0 & \Sigma_{ww} 
   \end{array} \right] \right)
\end{equation}

where $\Phi_f(.)$ is a $p$ th-order polynomial on $\mathbb{R}$ and $\Phi_u(.)$ is a $q$ th-order polynomial on $\mathbb{R}^{N \times N}$. In order to have identification, we assume $\Lambda:= [I,\Lambda'_2]'$ and $\Phi_u (.)$ and $\Sigma_{ww}$ diagonal. 

\begin{equation} \label{73}
\widehat{y}_{t_q} = \alpha + \beta \widehat{f}_{t_q}
\end{equation}

\bibliographystyle{apa}

\end{document}    

Answer 1

The culprit is breqn. The breqn package is known to have incompatibilities with other packages that deal with the mathematics environments, which means most of the ams* packages. For example:

• Why does adding \usepackage{breqn}, even if not used, cause LaTeX compile errors in some places?
• Error with \widetilde

The simple solution is to load breqn after loading any of the AMS packages. For example:

\documentclass[11pt]{article}

\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{hyperref}
\usepackage{amsfonts}

\usepackage{breqn}

\begin{document}

\begin{equation} \label{70}
\left( \begin{array}{c}
              v_{t_m} \\
              u_{t_m} 
   \end{array} \right) \sim N \left( 0, \left[ \begin{array}{cc}
              \Sigma_{vv} & 0 \\
              0 & \Sigma_{ww} 
   \end{array} \right] \right)
\end{equation}

where $\Phi_f(.)$ is a $p$ th-order polynomial on $\mathbb{R}$ and $\Phi_u(.)$ is a $q$ th-order polynomial on $\mathbb{R}^{N \times N}$. In order to have identification, we assume $\Lambda:= [I,\Lambda'_2]'$ and $\Phi_u (.)$ and $\Sigma_{ww}$ diagonal. 

\begin{equation} \label{73}
\widehat{y}_{t_q} = \alpha + \beta \widehat{f}_{t_q}
\end{equation}


\end{document} 

compiles without the duplication of y. But if you move \usepackage{breqn} before \usepackage{amsfonts} as you do in your example, you get the problem you showed in your post.