Overfull error in math mode?

Could someone explain this overfull error to me?

\documentclass[a4paper,12pt]{extarticle}
\usepackage[utf8]{inputenc}
\usepackage{amsthm}
\usepackage{amsmath}
\usepackage{amssymb}

\newtheorem{questioninner}{Question}
\newenvironment{question}[1]{%
\renewcommand\thequestioninner{#1}%
\questioninner
}{\endquestioninner}

\begin{document}

\begin{question}{2.1.35.1} Let $\mathsf{V}$ be a finite-dimensional vector space and $\mathsf{T \colon V \to V}$ be linear. Suppose that
$\mathsf{V = R(T) + N(T)}$. Then $\mathsf{V = R(T) \oplus N(T)}$.

\end{question}
\end{document}

• It is overfull of about 0.1 mm=1/10 mm ... You could ignore that overfull box or add a \linebreak command (at the point you would like the sentence to be breaked) when finalizing your document. Overfull boxes are not errors but warnings about failure of latex to not exceed the real text width ... But by default LaTeX leaves such small overfulls because it is not really a problem and possibly any other way would show worse. LaTeX "keeps in mind" some "penalties" (that allows or doesn't allows big differences from perfect fitted boxes) to break the lines that can be defined in the preamble – koleygr Aug 7 at 13:00
• @user_hello1: If you activate the draft mode by \documentclass[...,draft]{extarticle}, you will be able to see the problem in the resulting PDF. You can simply solve the problem by e.g. changing the margins. Simply add into your preamble \usepackage{geometry}. – Marian G. Aug 7 at 13:03
• Or use \hfuzz=length to specify when you want LaTeX to warn about overfull, I'll often use \hfuzz=3pt – daleif Aug 7 at 13:15
• Add “the map” before the map notation. Rewording is the most effective method for fixing overfull boxes. – egreg Aug 7 at 14:17
• @egreg: Sometimes, the issue can also be solved by loading the microtype package. Doing so in the concrete case presented by the asker, the warning disappears. – Marian G. Aug 7 at 14:36