I am writing Bibliography for an article and I have the following codes:

\begin{thebibliography}{9}

\bibitem{1}

Nathanial P.Brown and Narutaka Ozawa,

\textit{C*-Algebras and Finite-Dimensional Approximations},
Graduate studies in Mathematics, vol.88, American Mathematical Society, Providence, RI, 2008.

\bibitem{2}

P.de la Harpe and G.Skandalis,
\textit{Power's property and simple $C^*$-algebra},
Math.Ann.,273(1986),241-250.

\bibitem{3}

Uffe Haagerup,
\textit{A new look at $C^*$-simplicity and the unique trace property of a group},preprint(2015),arxiv:1509.05880.

\bibitem{4}

\textit{Stationary $C^*$-dynamical systems},preprint(2017),arXiv:1712.10133.
\end{thebibliography}


For (3) and (4) I get the following error messages: underfull \hbox(\badness 10000). I don't understand why these errors turn up. I wrote the exact same thing for (1) and (2) and there I didn't get any.

How do I fix this?

• Is it an error or a warning? – marmot Aug 4 '18 at 20:33
• The output for (3) and (4) is very different. There are more gaps between words. It is a warning. – tattwamasi amrutam Aug 4 '18 at 20:34
• You should consider adding space after commas, that allows LaTeX to shrink and stretch things a bit. You may also want to look at preprint(2017) (probably needs a space before the opening bracket), G.Skandalis (probably needs a ~ after the initial), Math.Ann. (needs space between Math. and Ann. whether \,, ~ or   you need to decide.) – moewe Aug 4 '18 at 20:38
• When I added space for (3), it worked out fine. But more space doesn't work for (4). – tattwamasi amrutam Aug 4 '18 at 20:41
• you have not really given enough information for anyone to help. The warning just means that tex has difficulty setting the text to the specified line width, but as you have not provided a usable example, we have no information about the text width or font size. – David Carlisle Aug 4 '18 at 20:53

You should consider adding a few more spaces here and there. Since LaTeX can break lines at spaces and can shrink and enlarge them to facilitate better line breaking that can already have a huge impact.

In particular when writing in English you'd normally leave a space - after a comma - before an opening bracket and after a closing bracket (unless followed by another punctuation) mark - after a dot (signifying an abbreviation) - after a full stop

You can avoid a line break at a space by using a tie ~.

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}

\begin{document}
\begin{thebibliography}{9}
\bibitem{brown}
Nathanial P. Brown and Narutaka Ozawa,
\textit{C*-Algebras and Finite-Dimensional Approximations},
Graduate studies in Mathematics, vol.~88, American Mathematical Society, Providence, RI, 2008.

\bibitem{harpe}
P. de la Harpe and G. Skandalis,
\textit{Power's property and simple $C^*$-algebra},
Math.~Ann. 273 (1986), 241--250.

\bibitem{haagerup}
Uffe Haagerup,
\textit{A new look at $C^*$-simplicity and the unique trace property of a group},
preprint (2015), arxiv:1509.05880.

\bibitem{hartman}
\textit{Stationary $C^*$-dynamical systems},

Note how I exchanged the meaningless numbers in \bibitem for more meaningful and more memorable keys.