Indent doesn't work inside enumerate enviroment

Question

I don't know why the 2nd line here is not indented. Have I loaded a package that overrides indentation? It seems not to work inside an enumerate enviroment.

\documentclass[12pt]{article}

\usepackage[utf8]{inputenc}
\usepackage{fullpage}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{graphicx}
\usepackage[backend=biber, isbn=false, doi=false]{biblatex-chicago}

\newtheorem{lem}{Lemma}
\newtheorem{ex}{Example}

\newcommand{\Q}{\mathbb{Q}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\N}{\mathbb{N}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\C}{\mathbb{C}}

\DeclareMathOperator{\lcm}{lcm}
\DeclareMathOperator{\ord}{ord}

\title{Homework 2 - MATH 4001}
\author{Clyde Kertzer}
\date{\today}

\begin{document}

\setlength{\abovedisplayskip}{5pt}
\setlength{\belowdisplayskip}{5pt}
\setlength{\abovedisplayshortskip}{0pt}
\setlength{\belowdisplayshortskip}{0pt}

\setlength{\parindent}{20pt}

\maketitle
\begin{enumerate}
\item[2.] Show that the sequence of functions
$$ f_n(x)=\frac{n x}{1+n x} $$ 
does not converge uniformly on $[0,1]$.

Paragraph 2 (Why no indent)?

Answer 1

That's the standard behavior in LaTeX. Whenever a list is opened, \listparindent is set to zero. A similar behavior have minipage and \parbox.

If you want to have a different parindent in lists, you have to specify it and the simplest way is to use enumitem.

\documentclass[12pt]{article}

\usepackage{enumitem}

\setlist[enumerate]{listparindent=\parindent}

\begin{document}

\begin{enumerate}
\item Show that the sequence of functions
\[
f_n(x)=\frac{n x}{1+n x}
\]
does not converge uniformly on $[0,1]$.

Paragraph 2 (Why no indent)?

\end{enumerate}

\end{document}

Answer 2

The following trick hides \parindent primitive register, so no macros have write access to this register. They can only read the value, which is copied to the faked \parindentA register.

\parindent=20pt  % setting desired value
\newdimen\parindentA  \parindentA=\parindent % value is copied  
\let\parindent=\parindentA  % the primitive \parindent is unaccessible after this
\expandafter \let \csname tex_parindent:D\endcsname=\parindentA