\documentclass[11p]{book}
\usepackage[T1]{fontenc}
\usepackage[american]{babel}
\usepackage[babel]{csquotes}
\usepackage[style=numeric,hyperref]{biblatex}
\usepackage{hyperref}
\usepackage{url}
\usepackage[center]{subfigure}
\usepackage{epsfig}
\usepackage{fullpage}
\renewcommand{\topfraction}{0.99}
\renewcommand{\bottomfraction}{0.99}
\renewcommand{\textfraction}{0}
\addtolength{\topmargin}{1cm}
\bibliography{lectures,hw1,hw2,hw3}
\defbibheading{bibliography}{%
  \section*{\refname}%
  \markboth{\MakeUppercase{\refname}}{\MakeUppercase{\refname}}}

\newenvironment{definition}[1][Definition]{\begin{trivlist}
\item[\hskip \labelsep {\bfseries #1}]}{\end{trivlist}}

\begin{document}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% YOUR TEXT HERE
%
% ONLY TEXT IN THIS AREA SHOULD 
% BE SUBMITTED FOR THE BOOK
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% print all cites mentioned in the file
\printbibliography

\end{document}