# \blacksquare in .sty file

I am planning to have a .sty file for scibing notes. When modifying one that already exists online, I found that I could not use

 \renewcommand{\qedsymbol}{\blacksquare}


or

 \renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}


either to change the usual white square to the black one. So, how would I do this?

On a similar note, if I wanted to add a blacktriangle, how would I do this?

In case, the answer depends on how the rest of the .sty file looks, I am most willing to point to the file I am modifying.

## My .sty file:

 \newtheoremstyle{myplain}      {10pt}{10pt}{\itshape}{}{\scshape}{.}{.5em}{}
\newtheoremstyle{mydefinition} {10pt}{10pt}{}{}{\scshape}{.}{.5em}{}
\newtheoremstyle{myremark}     {10pt}{10pt}{}{}{\scshape}{.}{.5em}{}

\renewcommand{\qedsymbol}{\blacksquare}   %replacing this line with
%\renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}
% does not work either.

\newcounter{lecture}
\theoremstyle{myplain}
\newtheorem{thm}{Theorem}[lecture]
\newtheorem{lem}         [thm]{Lemma}
\newtheorem{pro}         [thm]{Proposition}
\newtheorem{cor}         [thm]{Corollary}
\newtheorem{claim}       [thm]{Claim}
\newtheorem{fact}        [thm]{Fact}

\theoremstyle{mydefinition}
\newtheorem{defn}       [thm]{Definition}
\newtheorem{eg}         [thm]{Example}
\newtheorem{assumption} [thm]{Assumption}
\newtheorem{openproblem}[thm]{Open Problem}
\newtheorem{problem}    [thm]{Problem}

\theoremstyle{myremark}
\newtheorem{remark}        [thm]{Remark}
\newtheorem{conjecture}    [thm]{Conjecture}

\numberwithin{equation} {lecture}
\numberwithin{figure}   {lecture}
\numberwithin{table}    {lecture}

\renewcommand{\fnum@figure}{\textsc{Figure~\thefigure}}
\renewcommand{\fnum@table}{\textsc{Table~\thetable}}

\renewcommand{\thesection}{\thelecture.\arabic{section}}
\renewcommand{\thepage}{\thelecture-\arabic{page}}

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I am sure my tags are off. Will someone please fix it for me? – kan May 13 '12 at 11:59
Would you please add a MWE? – egreg May 13 '12 at 12:30
Sure, I'll do @egreg – kan May 13 '12 at 12:47

You probably forgot to load amssymb:

\documentclass{amsart} % just to have amsthm preloaded
\usepackage{amssymb}
\renewcommand{\qedsymbol}{$\blacksquare$}
\begin{document}
\begin{proof}
a
\end{proof}
\end{document}

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Oh, OK. It works. I have to load amssymb before loading my .sty file and use \ensuremath{}! – kan May 13 '12 at 13:17
@KannappanSampath You don't have to use \ensuremath: use $\blacksquare$, which is the correct way. – egreg May 13 '12 at 13:55