Writing infinity symbol(∞) in LaTeX

$\infty$
\infty

Use the stix package in your document to use below LaTeX symbols

\usepackage{stix}

infinity
$\infty$
\infty

Incomplete infinity
⧜
\iinfin

Tie over infinity
⧝
\tieinfty

Infinity negated with vertical bar
⧞
\nvinfty

Permanent paper sign
♾
\acidfree

Use the graphicx package in your document to use below LaTeX symbols

\usepackage{graphicx}

$\infty$
\def\infinity{\rotatebox{90}{8}}
$ \infinity $