How to use less-than or equivalent to symbol in LaTeX?

Symbol \| Unicode	Less-Than or Equivalent To \| U+2272
Type of symbol	Mathematics
Package (requirement)	amssymb
Argument	No
Command	`\lesssim`
Example	`\lesssim` → ≲

There is no default command to print the less-than or equivalent to symbol to a LaTeX document.

For this you need the amssymb package, this package provides a command called \lesssim which allows you to print this symbol in the document.

\documentclass{article}
\usepackage{amssymb}
\begin{document}
   \[ a \lesssim b \]
   \[ x_1 \lesssim x_2 \]
   \[ \frac{1}{a_1} \lesssim \frac{1}{a_2} \]
\end{document}

Output :