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l.51 begin{document}

    The control sequence at the end of the top line
    of your error message was never def'ed. If you have
    misspelled it (e.g., `hobx'), type `I' and the correct
    spelling (e.g., `Ihbox'). Otherwise just continue,
    and I'll forget about whatever was undefined.

%----------------------------------------------------------------------------------------
    %   VARIOUS REQUIRED PACKAGES AND CONFIGURATIONS
    %----------------------------------------------------------------------------------------

    usepackage[top=3cm,bottom=3cm,left=3cm,right=3cm,headsep=10pt,a4paper]{geometry} % Page margins

    usepackage{graphicx} % Required for including pictures
    graphicspath{{Pictures/}} % Specifies the directory where pictures are stored

    usepackage{lipsum} % Inserts dummy text

    usepackage{tikz} % Required for drawing custom shapes

    usepackage[english]{babel} % English language/hyphenation

    usepackage{enumitem} % Customize lists
    setlist{nolistsep} % Reduce spacing between bullet points and numbered lists

    usepackage{booktabs} % Required for nicer horizontal rules in tables

    usepackage{xcolor} % Required for specifying colors by name
    definecolor{ocre}{RGB}{243,102,25} % Define the orange color used for highlighting throughout the book

    %----------------------------------------------------------------------------------------
    %   FONTS
    %----------------------------------------------------------------------------------------

    usepackage{avant} % Use the Avantgarde font for headings
    %usepackage{times} % Use the Times font for headings
    usepackage{mathptmx} % Use the Adobe Times Roman as the default text font together with math symbols from the Sym­bol, Chancery and Com­puter Modern fonts

    usepackage{microtype} % Slightly tweak font spacing for aesthetics
    usepackage[utf8]{inputenc} % Required for including letters with accents
    usepackage[T1]{fontenc} % Use 8-bit encoding that has 256 glyphs

    %----------------------------------------------------------------------------------------
    %   BIBLIOGRAPHY AND INDEX
    %----------------------------------------------------------------------------------------

    usepackage[style=alphabetic,citestyle=numeric,sorting=nyt,sortcites=true,autopunct=true,babel=hyphen,hyperref=true,abbreviate=false,backref=true,backend=biber]{biblatex}
    addbibresource{bibliography.bib} % BibTeX bibliography file
    defbibheading{bibempty}{}

    usepackage{calc} % For simpler calculation - used for spacing the index letter headings correctly
    usepackage{makeidx} % Required to make an index
    makeindex % Tells LaTeX to create the files required for indexing

    %----------------------------------------------------------------------------------------
    %   MAIN TABLE OF CONTENTS
    %----------------------------------------------------------------------------------------

    usepackage{titletoc} % Required for manipulating the table of contents
%----------------------------------------------------------------------------------------
%   PAGE HEADERS
%----------------------------------------------------------------------------------------

usepackage{fancyhdr} % Required for header and footer configuration

pagestyle{fancy}
%----------------------------------------------------------------------------------------
%   THEOREM STYLES
%----------------------------------------------------------------------------------------

usepackage{amsmath,amsfonts,amssymb,amsthm} % For math equations, theorems, symbols, etc

%----------------------------------------------------------------------------------------
%   DEFINITION OF COLORED BOXES
%----------------------------------------------------------------------------------------

RequirePackage[framemethod=default]{mdframed} % Required for creating the theorem, definition, exercise and corollary boxes

%----------------------------------------------------------------------------------------
%   HYPERLINKS IN THE DOCUMENTS
%----------------------------------------------------------------------------------------

usepackage{hyperref}
hypersetup{hidelinks,backref=true,pagebackref=true,hyperindex=true,colorlinks=false,breaklinks=true,urlcolor= ocre,bookmarks=true,bookmarksopen=false,pdftitle={Title},pdfauthor={Author}}
usepackage{bookmark}