TexStudio and TexLive 2021 in Custom Linux OS "Package inputenc error: Invalid UTF-8 byte sequence"

Question

I want to know how to make the symbol can be compiled and shown?

Thus I can use symbol of omega instead of writing $\omega$, or symbol of pi instead of $\pi$.

I am able to do this and compile it in other Linux OS (CAELinux) with TexLive version 2017, but it can't be done on this OS with TL version 2021. What package or setting do I need to adjust?

this is the MWE:

\documentclass[twoside]{book}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{amsfonts}
\usepackage{latexsym}
\usepackage{enumerate}
\usepackage{wrapfig}
\usepackage{siunitx}
\usepackage{cite}
\usepackage{cancel}
\usepackage{ulem}
\usepackage{makecell}
\usepackage[utf8]{inputenc}

\begin{document}
\begin{equation}
        \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}} = 0 \\
        a^{2} \left( \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}}  \right) = \frac{∂w}{∂t} \\
        a^{2} \left( \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}}  \right) = \frac{∂^{2}w}{∂t^{2}} 
\end{equation}

\end{document}

Answer 1

The important part of the output is:

! LaTeX Error: Unicode character ∂ (U+2202)
               not set up for use with LaTeX.

There are two ways to fix this. First, if you can use LuaLaTeX or XeLaTeX, the unicode-math package does understand every Unicode math symbol. (PDFlaTex only understands characters from the legacy 8-bit encodings that have been loaded.) Removing some packages that are redundant with unicode-math, this works:

\documentclass[twoside]{book}
\usepackage{mathtools}
\usepackage{unicode-math}
\usepackage{enumerate}
\usepackage{wrapfig}
\usepackage{siunitx}
\usepackage{cite}
\usepackage{cancel}
\usepackage{ulem}
\usepackage{makecell}

\begin{document}
\begin{equation}
        \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}} = 0 \\
        a^{2} \left( \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}}  \right) = \frac{∂w}{∂t} \\
        a^{2} \left( \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}}  \right) = \frac{∂^{2}w}{∂t^{2}} 
\end{equation}

\end{document}

If you are required to use PDFLaTeX, you need to set up that character. One way to do this is with newunicodechar.

\documentclass[twoside]{book}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{latexsym}
\usepackage{enumerate}
\usepackage{wrapfig}
\usepackage{siunitx}
\usepackage{cite}
\usepackage{cancel}
\usepackage{ulem}
\usepackage{makecell}
\usepackage{newunicodechar}
\usepackage[utf8]{inputenc}

\newunicodechar{∂}{\partial}

\begin{document}
\begin{equation}
        \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}} = 0 \\
        a^{2} \left( \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}}  \right) = \frac{∂w}{∂t} \\
        a^{2} \left( \frac{∂^{2}w}{∂x^{2}} + \frac{∂^{2}w}{∂y^{2}} + \frac{∂^{2}w}{∂z^{2}}  \right) = \frac{∂^{2}w}{∂t^{2}} 
\end{equation}

\end{document}

If you also want to be able to use ∂ in text mode, not $∂$, you would change the command to

\newunicodechar{∂}{\ensuremath{\partial}}

But I recommend against this.