How to make a box bold at the end of proof? [duplicate]

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}{}}

\fancyfoot{}
\fancyhead[RE]{\slshape Basic Algebraic Topology and its Applications}

\newtheoremstyle{myplain}% name
{3pt}% Space above
{3pt}% Space below
{\itshape}% Body font
{}% Indent amount
{}% 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
{}% 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 :

But I would like to produce :

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. VerronMay 31 '13 at 18:44

\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

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>:

\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}}


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