[Tex/LaTex] Undefined control sequence with empheq package

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I am new in this community and TeX. I am trying to add some equations in my TeX file with empheq package, I assume something is going on with the installation of the package, I am using Kile KDE editor.

These are the packages used:

     %% packages
    \usepackage[T1]{fontenc}
    \usepackage[utf8x]{inputenc}
    \usepackage{float}
    \usepackage{amsfonts,amssymb} % needed for math fonts
    %\usepackage[ruled,vlined,linesnumbered,resetcount,algochapter]{algorithm2e}
    \usepackage{amsmath} % needed for command eqref
    %\usepackage{mathtools}
    %\usepackage{algorithm}
    %\usepackage{algorithmic}
    %\usepackage{algorithmic}
    \usepackage{longtable}
    \usepackage{lmodern}
    %\usepackage{bookmark}
    %\usepackage{algpseudocode}
    %\usepackage{algcompatible}
    %
    \usepackage[greek,english]{babel}
    \usepackage{alphabeta}

    \usepackage[
        unicode
        ,colorlinks=true
        ,breaklinks
        %,greek
        ]{hyperref} % needed for creating hyperlinks in the document, the option colorlinks=true gets rid of the %awful %boxes, breaklinks breaks lonkg links (list of figures), and ngerman sets everything for german as %default hyperlinks %language
    %\usepackage[utf8]{inputenc}
    %\usepackage{bookmark}
    \usepackage{algpseudocode}

    %\usepackage[hyphenbreaks]{breakurl} % benötigt für das Brechen von URLs in Literaturreferenzen, hyphenbreaks auch bei %links, die über eine Seite gehen (mit hyphenation).
    \usepackage{xcolor}
    %\definecolor{c1}{rgb}{0,0,1} % blue
    %\definecolor{c2}{rgb}{0,0.3,0.9} % light blue
    %\definecolor{c3}{rgb}{0.3,0,0.9} % red blue%
    \hypersetup{
      colorlinks=True,
        linkcolor=blue, % internal links    
        citecolor=green!0.3!blue!0.9,
      urlcolor=red!0.3!blue!0.9} % external links/urls

    \usepackage{alphabeta}
\usepackage{algorithm, algcompatible}
\usepackage{empheq, nccmath} 
    %\usepackage{cite}          % needed for cite
    \usepackage[nottoc]{tocbibind}      % needed for displaying bibliography and other in the table of contents
    \usepackage[numbers, authoryear]{natbib}    % needed for cite and abbrvnat bibliography style
    \usepackage{graphicx}           % needed for \includegraphics 
    \usepackage{bigstrut}           % needed for the command \bigstrut
    \usepackage{enumerate}          % needed for some options in enumerate
    %\usepackage{todonotes}         % needed for todos
    %\usepackage{makeidx}           % needed for creating an index
    %\makeindex
    \newcommand\Mycite[1]{%
      \citeauthor{#1}~[\citeyear{#1}]}

This is the main TeX script:

\documentclass[a4paper, 12pt, openany]{book}

    \input{settings/packages}
    %\usepackage{algorithm, algcompatible}
    %\usepackage{empheq, nccmath} 
    \algnewcommand\INPUT{\item[\textbf{Input:}]}%
    \algnewcommand\OUTPUT{\item[\textbf{Output:}]}%

    %\input{settings/page}
    \graphicspath{{Images/}}
    \DeclareMathOperator*{\argmax}{argmax}
    %\makeglossaries

    %\input{settings/macros}
    %\newcommand{\imp}[1]{\underline{\textit{#1}}}
    %\input{Contents/abbreviations}
    \bibliographystyle{abbrvnat}
    %\setcitestyle{}
    %\setcounter{page}{4} 
    % Main document Below
    \begin{document}

    \pagestyle{empty}
    %\input{Contents/title_page} 
    \input{abstractGR}
    %\input{abstractEn}

    \pagestyle{plain}
    %\setotherlanguage{greek}
    %\selectlanguage{greek}
    \listoffigures
    \listoftables
    \tableofcontents
    \raggedbottom
    %\flushbottom
    %\clearpage\phantomsection
    %\selectlanguage{english}

    \pagebreak
    %\printglossary[type=\acronymtype,title=Abbreviations]
    % Insert dedication
    \vspace*{10cm}
    \centering{\large{\emph{\textgreek{ Στην Οικογενειά μου...} }}}
    \vfill
    \pagebreak

    \raggedright
    %\pagestyle{plain}
    %\input{chapters/introduction}
    %\input{chapters/modelSelection}
    %\input{chapters/post_process}
    %\chapter{}
    %\input{chapters/experiments}
    %\input{chapters/conclusion}
    %\appendix
    %\addtocontents{toc}{\contentsline{chapter}{\numberline{}APP}{}{}}
    %\addtocontents{toc}{\protect\contentsline{chapter}{Appendix}{}}
    %\input{chapters/appendix}
    %\addtocontents{toc}{\protect\contentsline{chapter}{\protect\numberline{}Appendix}{}{chapter*.\thepage}}

        %\newpage
        %\chapter{Introduction}

        \medskip
        %\citet*{*}
        %\cite{latexcompanion}
        %\bibliography{thesis}

        %\printindex

    \end{document}

abstractGR.tex file that includes empheq

\chapter*{\centering \textgreek{Περίληψη}}
%\addtocontents{toc}{\protect\contentsline{chapter}{\protect\numberline{}\textgreek{Περίληψη (Ελληνικά)}}{}{chapter*.%\thepage}}
%\addcontentsline{toc}{chapter}{Abstract (Greek)}
\textgreek{Η παρούσα διπλωματική εξετάζει το πρόβλημα της αναγνώρισης αντικειμένων από εικόνες, των οποίων τα πίξελ είναι ταξινομημένα σε μια από τις 19 κατηγορίες. Η εργασία χρησιμοποιεί μια βάση δεδομένων που αποτελείται από 19 διαφορετικές κατηγοριές αντικειμένων η οποία έχει δημιουργηθεί με χρήση κάμερας τοποθετημένη στο εμπρόσθιο μέρος του αυτοκινήτου. Οι εικόνες έχουν απαθανατιστεί από 50 διαφορετικές πόλεις της Ευρώπης σε διάφορες εποχές και καιρικές συνθήκες.\\

Με την χρήση πληροφορίας από εικόνες κατασκευάζουμε έναν ταξινομηττή ο οποίος μπορεί να αναγνωρίσει την κατηγορία αντικειμένων που ανήκει το κάθε πίξελ στην εικόνα ως συνάρτηση του μεγέθους των pixel αλλά και της δομής που απεικονίζουν. Για την ταξινόμηση χρησιμοποιήσαμε 2 πανομοιότυπα μοντέλα πλήρως συνελικτικών νευρωνικών δικτύων (}FCNNs\textgreek{) σε συνδυασμό με 2 διαφορετικά μοντέλα μετά επεξεργασίας. Το πρώτο μοντέλο κάνει χρήση του Μέσου Φίλτρου(}Median Filter\textgreek{), ενώ το δεύτερο κάνει χρήση μιας παραλλαγής του στατιστικού μοντέλου }CRF. \textgreek{Για την εικονοποίηση των παραπάνω αποτελεσμάτων υλοποιήθηκε λογισμικό που απεικονίζει τα αποτελέσματα των μεθόδων. Για την κατασκευή των παραπάνω μοντέλων γίνεται χρήση των βιβλιοθηκών }Scikit-learn, Keras \textgreek{ και} Tensorflow, \textgreek{ ενώ για την υλοποίηση του λογισμικού εικονοποίησης έγινε η χρήση της βιβλιοθήκης} pyQt.


\chapter*{\centering Abstract}
%\addtocontents{toc}{\protect\contentsline{chapter}{\protect\numberline{}Abstract (English)}{}{chapter*.\thepage}}

This thesis focuses the problem of recognizing objects from images whose images are classified in one of the 19 categories. The job uses a database consisting of 19 different categories of objects created using a camera located on the front of the car. Images have been recorded in 50 different European cities in different seasons and weather conditions.

Using information from images, we construct a classifier that can recognize the category of objects belonging to each image of the image as a function of the size of the pixels and the structure they represent. For classification, we used 2 identical Fully Convolutional Neural Networks in combination with 2 different models after editing. The first model uses Median Filter), while the latter uses a variation of the CRF statistical model. To illustrate the above results, we have implemented software that illustrates the results of the methods. To build the above models, we use Scikit-learn, Keras and Tensorflow, while using pyQt libraries.

\begin{empheq}{align} 
    \mathit{Loss(P,Q)} &= - \frac{1}{N} \sum_{x}^{N} P(x)\times 
    \log(Q(x)) \times \alpha_{\mathit{coefficients}}\\
    \nabla\times\overline{E}\notag
     &= -\frac{\partial\overline{B}}{\partial t}-\bar J_{\text{mi}} 
\end{empheq}

The empheq call is in the chapter abstractGR.tex from the one above, if I move the packages from the packages.tex to main.tex works like a charm.

Best Answer

UPDATE: Document class book works fine.

\documentclass{book}
\usepackage{empheq}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}
    \begin{empheq}[left=\empheqlbrace]{align}
    \mathit{Loss(P,Q)} &= - \frac{1}{N} \sum_{x}^{N} P(x)\times 
    \log(Q(x)) \times \alpha_{\mathit{coefficients}}\\
    \nabla\times\overline{E}\notag
     &= -\frac{\partial\overline{B}}{\partial t}-\bar J_{\text{mi}} 
\end{empheq}
\end{document}

enter image description here

I stole the {\mathit{coefficients} from @Zarko and replaced \bar by \overline because IMHO this looks better.

SECOND UPDATE: Your code had several option clashes. I think I solved all of them apart from those associated to babel and greek, which I never was able to run on my machine. For instance, you do not need definecolor in order to set the link etc. colors.

\documentclass{book}
%% packages
\usepackage{amsfonts,amssymb} % needed for math fonts
\usepackage{empheq, nccmath}
\usepackage{amsmath} % needed for command eqref
\usepackage{longtable}
\usepackage{lmodern}
\usepackage{xcolor}
\usepackage[unicode,
    ,colorlinks=true
    ,breaklinks
%    ,greek
    ]{hyperref}
\usepackage{bookmark}
\hypersetup{
  colorlinks=true,
  linkcolor=blue,
  citecolor=green!0.3!blue!0.9
  urlcolor=red!0.3!blue!0.9}
%\usepackage[greek,english]{babel}
\usepackage{alphabeta}
%%%
% \usepackage{} 
%\usepackage[utf8]{inputenc}
\usepackage{algpseudocode}
\usepackage{algorithm}
%\usepackage[hyphenbreaks]{breakurl} % benötigt für das Brechen von URLs in Literaturreferenzen, hyphenbreaks auch bei %links, die über eine Seite gehen (mit hyphenation).
 % needed for creating hyperlinks in the document, the option colorlinks=true gets rid of the %awful %boxes, breaklinks breaks lonkg links (list of figures), and ngerman sets everything for german as %default hyperlinks %language
% \definecolor{c1}{rgb}{0,0,1} % blue
% \definecolor{c2}{rgb}{0,0.3,0.9} % light blue
% \definecolor{c3}{rgb}{0.3,0,0.9} % red blue
% \hypersetup{
%     linkcolor={c1}, % internal links
%     citecolor={c2}, % citations
%     urlcolor={c3} % external links/urls
% }

%

\usepackage{alphabeta}
\usepackage[numbers, authoryear]{natbib}    % needed for cite and abbrvnat bibliography style
\usepackage[nottoc]{tocbibind}      % needed for displaying bibliography and other in the table of contents
\usepackage{graphicx}           % needed for \includegraphics 
\usepackage{bigstrut}           % needed for the command \bigstrut
\usepackage{enumerate}          % needed for some options in enumerate
% \newcommand\Mycite[1]{%
%   \citeauthor{#1}~[\citeyear{#1}]}
% 

    \begin{document}        
    \begin{empheq}[left=\empheqlbrace]{align}
            \mathit{Loss(P,Q)} = - \frac{1}{N} \sum_{x}^{N} P(x)\times log(Q(x)) \times \alpha_{coefficients}
            \nabla\times\bar E &= -\frac{\partial\bar B}{\partial t}-\bar J_{\text{mi}} 
        \end{empheq}
    \end{document}


    \documentclass{book}
\usepackage{empheq}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}
    \begin{empheq}[left=\empheqlbrace]{align}
    \mathit{Loss(P,Q)} &= - \frac{1}{N} \sum_{x}^{N} P(x)\times 
    \log(Q(x)) \times \alpha_{\mathit{coefficients}}\\
    \nabla\times\overline{E}\notag
     &= -\frac{\partial\overline{B}}{\partial t}-\bar J_{\text{mi}} 
\end{empheq}
\end{document}

enter image description here

I did not massage the equation here because I'm afraid of @Zarko ;-)

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