is there a way to modify the formatting of chapter or section headings to have them centered ?
Try the fancyheader package (fancyhdr) and use something like:
\usepackage{fancyhdr}
\pagestyle{fancy}
\chead{\chaptermark}
I'm new to this package so I'm sure you can get better results if you read
the documentation through...
Hope this works, regards - Diego
