I am using Overleaf to type up a paper involving proof of different results and I would like to write the box at the end of a proof that means QED ie end of proof. How can I do that? Thanks.
I am using
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
As @egreg said in the comments, you can use the proof environment from amsthm, which will automatically place a QED symbol at the end of the proof. You can also use the command \qedsymbol, also from amsthm, to place a QED symbol anywhere. Here's an example of both options, with the preamble you provided.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\begin{document}
\begin{proof}
A proof written in the \verb|proof| environment. The QED symbol will appear at the end.
\end{proof}
A \qedsymbol{} outside of a \verb|proof| environment.
\end{document}
In fact, amsthm uses the command \qedsymbol to place the symbol at the end of the proof environment, so if you want to use something else as a QED symbol (for example a black square), you can simply redefine \qedsymbol, and the proof environment will use it.
\usepackage{amsthm}and use\begin{proof}...\end{proof}for the proofs. The QED symbol will magically appear. – egreg Sep 29 '20 at 21:09amsmathbeforeamsthm. Otherwise the QED symbol can appear out of place in some environments. See this question for example. – Sandy G Sep 29 '20 at 21:53