Problem using enumerate environment in a proof newenvironment definition

Question

I want to define a simple environment to enclose my proofs, actually I just wanted to put a little square right at the end. So, I made this new environment definition:

\newenvironment{Proof}{ \flushleft \textbf{PROOF}}{\rule{1ex}{1ex}}

When I use this environment to enclose simple text, then I get the little square right at the end, after the last character. The problem is when I enclose an enumerate environment because it adds a new line and put the little square at the beginning. In this case my problem is both I don't want the new line and also I need the square at the right ending of the margin. If I consider it this way

\newenvironment{Proof}{ \flushleft \textbf{PROOF}}{\begin{flushright} \rule{1ex}{1ex} \end{flushright}}

then a new line is always added.

Answer 1

amsthm automatically puts a box (an open box, but that can be redefined) flush right at the end of the last line of proof. if the proof ends with a display or a list, then \qedhere can be used just before the end of that environment to move the box to the proper line.

to start the enumerated list on a new line after the proof header, enter it as

\begin{proof}
 \leavevmode
 \begin{enumerate}
 ...

documentation is available with texdoc amsthm (if you are using a system based on tex live) or by visiting the page and clicking on the line "AMS theorem package user's guide".

to redefine the qed box,

\renewcommand{\qedsymbol}{...}

to change the main proof header,

\renewcommand{\proofname}{Whatever}

(thanks, egreg.)

for a one-time variant, just take advantage of the optional argument:

\begin{proof}[Proof of the XYZ theorem}