what can make \subsection now show up in article document? No matter what setting I do, such as \setcounter{tocdepth}{3}, or \setcounter{tocdepth}{4} the subsections do not show up in toc. All my subsection have such header:
\section{Integrals}
\subsection{\hspace{5 mm} Integral No. 1}
...
\subsection{\hspace{5 mm} Integral No. 2}