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Let us consider the following example :

\documentclass[12pt,twoside,a4paper]{book}
\usepackage[left=.45in,right=.45in,top=.6in,bottom=.6in]{geometry}
\usepackage[fleqn]{amsmath} 
\usepackage{amsfonts}

\usepackage{amsthm,amssymb,graphicx}
\usepackage[all]{xy}
\usepackage{graphicx,wrapfig,tikz}
\usepackage{enumerate}
\usepackage{fancyhdr}


 \usetikzlibrary{positioning,decorations.markings}


\pagestyle{fancy}

\renewcommand{\chaptermark}[1]{\markboth{#1}{}}

\fancyhead{}
\fancyfoot{}
\fancyhead[LE,RO]{\thepage}
\fancyhead[LO]{\slshape\leftmark } 
\fancyhead[RE]{\slshape Basic Algebraic Topology and its Applications}   




\newtheoremstyle{myplain}% name
{3pt}% Space above
{3pt}% Space below
{\itshape}% Body font
{}% Indent amount
{\bfseries}% Theorem head font
{}% Punctuation after theorem head
{.5em}% Space after theorem head
{}% Theorem head spec (can be left empty, meaning `normal')


\theoremstyle{myplain}

\newtheorem{thm}{Theorem}[section]

\newtheorem{prop}[thm]{Proposition}

\newtheorem{defn}[thm]{Definition}
\newtheorem{example}[thm]{Example}
\newtheorem{lemma}[thm]{Lemma}


\newtheoremstyle{mytheoremm}% name
{3pt}% Space above
{3pt}% Space below
{\itshape}% Body font
{}% Indent amount
{\bfseries}% Theorem head font
{.}% Punctuation after theorem head
{.5em}% Space after theorem head
{}% Theorem head spec (can be left empty, meaning `normal')

\theoremstyle{mytheoremm}
\newtheorem{remark}{Remark}
\newtheorem*{remark*}{Remark}

\newtheorem{cor}{Corollary}
\newtheorem*{cor*}{Corollary}

\begin{document}
\setcounter{page}{0}\mbox{}\newpage
 \thispagestyle{plain}
 \def\chapternum{2}
\setcounter{chapter}{1}

\chapter{Homotopy}
\begin{thm}
 Text.,...
\end{thm}
\begin{proof}
 Text Text  Text Text
\end{proof}

 \end{document}

This gives :

enter image description here

But I would like to produce :

enter image description here

i.e., the box should more bold. How can I do so?

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marked as duplicate by Kurt, mafp, Claudio Fiandrino, cmhughes, T. Verron May 31 '13 at 18:44

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2  
\renewcommand{\qedsymbol}{<whatever>} –  Werner May 31 '13 at 16:27
    
You can use some hints from How to change the end-proof symbol in Beamer? in combination with Werner's comment. –  Claudio Fiandrino May 31 '13 at 16:31
    
@Werner,thank you. +1 for you. –  Soumitra Sen May 31 '13 at 16:35

2 Answers 2

up vote 3 down vote accepted

amsthm sets the box at the end of the proof environment using as \qedsymbol. The default for \qedsymbol is \openbox which is defined as

\newcommand{\openbox}{\leavevmode
  \hbox to.77778em{%
  \hfil\vrule
  \vbox to.675em{\hrule width.6em\vfil\hrule}%
  \vrule\hfil}}

The rules (\hrule and \vrule) in this box is .4pt.

In general, using \renewcommand{\qedsymbol}{<whatever>} will switch this symbol to <whatever>. Here's an update to \openbox in the form of \OpenBox[<len>] that sets a box similar to \openbox but with a line thickness of <len>:

enter image description here

\documentclass{article}
\usepackage{amsthm}% http://ctan.org/pkg/amsthm
\newcommand{\OpenBox}[1][.4pt]{\leavevmode
  \hbox{% to.77778em{%
  \hfil\vrule width#1
  \vbox to.675em{\hrule width.6em height#1\vfil\hrule height#1}%
  \vrule width#1\hfil}}
\begin{document}
\begin{minipage}{0.4\linewidth}

\begin{proof} This is a proof. \end{proof}

\renewcommand{\qedsymbol}{\OpenBox[.8pt]}

\begin{proof} This is a proof. \end{proof}

\renewcommand{\qedsymbol}{\OpenBox[1pt]}

\begin{proof} This is a proof. \end{proof}

\renewcommand{\qedsymbol}{\OpenBox[2pt]}

\begin{proof} This is a proof. \end{proof}
\end{minipage}
\end{document}

Of course the last one isn't pretty, but it's meant to show you the implications of thickening the rules. The box isn't perfectly square. However, if you have a fixed rule width you're working with, the parameters inside \OpenBox can be tweaked to make the box square.

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Your box is rather big, so maybe something like the following (A to see the proportions. All into a definition after setting up the dimensions):

A \rule{1mm}{4mm}\hspace{-1mm}\rule{4mm}{1mm}\hspace{-1mm}\rule{1mm}{4mm}\hspace{-4mm}\raisebox{3mm}{\rule{4mm}{1mm}}

enter image description here

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