Some paragraphs in my document do not indent, what ever I try. I have solved some of them using the \parindent and \usepackage{indentfirst} but these specific ones do not seem to work. In those specific paragraphs, I include some math environments. Following each paragraph is a math environment on its own line. Can someone tell me how to fix this?
Here is the code and subsequent pdf view:

The added code at the beginning of the document:

\documentclass, the indentation may be set to zero by default. Add, for example,\parindent=15pt\relaxto your preamble and see what happens upon recompilation. – Steven B. Segletes Jun 21 '17 at 11:55$$ ... $$in LaTeX (https://tex.stackexchange.com/q/503). For single line equations, use\[ ... \], for multiline equations without alignment (like your first two equation blocks, add\usepackage{amsmath}if you haven't already, and use\begin{gather*} first line \\ second line \\ third line \end{gather*}. – Torbjørn T. Jun 21 '17 at 12:00$$for display math in latex) you appear to have parindent set to 0pt. but as you have shown no code it is hard to be sure. – David Carlisle Jun 21 '17 at 12:07