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Why does this line causes undefined control sequence error?
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\title{Amsmath example}
\author{ShareLaTeX Templates}
\date{June 2017}
\begin{document}
\maketitle
\section{Introduction}
$c \in \interval[{3q/8, 7q/8})$
\end{document}
Just reading the guide to the pag. 3, of interval package as written in the @Bernard's comment. http://ctan.mirror.garr.it/mirrors/CTAN/macros/latex/contrib/interval/interval.pdf.
You can see also the bad result when you use \dfrac option.
\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath,amssymb}
\usepackage{interval}
\title{Amsmath example}
\author{ShareLaTeX Templates}
\date{\today}
\begin{document}
\maketitle
\section{Introduction}
$c \in \interval{3q/8}{7q/8}$
\vskip 1cm
$c \in \interval{\frac{3q}{8}}{\frac{7q}{8}}$
\vskip 1cm
$c \in \interval{\dfrac{3q}{8}}{\dfrac{7q}{8}}$
\end{document}
\interval(or causes it to be defined). So the error we get is pretty natural. Maybe you were hoping to use the command from tex.stackexchange.com/a/135529/35864? In that case you just forgot to include the relevant code in your preamble.intervalpackage, but the syntaxt uses two arguments