Where exactly is it not showing up? I see subsection "0.2.1 Mathematical" both in the text and in the table of contents (you need to compile twice or three times for it to make its way to the table of contents).
I would also suggest leaving a blank line to separate paragraphs instead of using \\. This indents the paragraph's first line and inserts the proper vertical space between paragraphs. Also, it gives the source code a structure that can be recognized and used by text editors.