How to put the headline in the center of line, not from the left side of line? when I used \begin{center} \section{abstract} \end{center} It brought me a lot of space under the headline. Is there any convenient way to do that? Also, when I used that kind of way to put headline in the center and when I compiled it, it complained about the line at \end{center}. Is there any professional way to do that? Thanks a lot.