How could I "adjust" (in authomatic way) the way in which a reference to an appendix result appears? Could that be customized? (A.1 vs 1)

Question

In the following lines of code (a MWE), I intend to have appendix results named like A.1, A.2, etc. (see line "\newtheorem{APP.}{A.}").

\documentclass[12pt,oneside,english]{report} 

\usepackage[a4paper]{geometry}
\geometry{verbose, tmargin=2cm, bmargin=2cm, lmargin=2cm, rmargin=2cm, headsep=0.5cm, footskip=1cm}
\usepackage[hidelinks]{hyperref}
\newtheorem{APP.}{A.}

\begin{document}

\tableofcontents
  
\chapter{FFFFFFFFFFFFFFFFF}\label{chap: Brownian motion theory}
\justify 
\pagenumbering{arabic}
\lhead{\textbf{\rightmark}}
\section{WWWWWW}\label{subsec: History of Brownian motion}
\justify 
As pointed out in \ref{app.:law XXXXXX}...


\chapter{Appendix} \label{app: Appendix ACCA}
\begin{APP.}\textbf{(Law XXXXXX)}\label{app.:law XXXXXX}
\noindent According to ...

\end{APP.}
    
\end{document}

Appendix part will appear as:

Hence, as intended, first appendix result will be named A.1.

---

---

With the line:

 As pointed out in \ref{app.:law XXXXXX}...

I get the output:

As pointed out in 1

While I would like to get instead:

As pointed out in A.1

---

---

---

I understand the point and the reason of that. Is there any possible authomatic way (I know how to do it manually, but it is too time-expensive) to get A.1 instead of 1 in the body text reference? Should I make some adjustment as to the way in which I define the appendix (which currently corresponds to "\newtheorem{APP.}{A.}") or should I make any adjustment somewhere else?

Answer 1

When I process your MWE with pdflatex there are several problems including "undefined control sequence" and a hyperref problem.

In the code below I have commented out the problematic lines.

% newtheoremprob.tex  SE 568951

\documentclass[12pt,oneside,english]{report} 

\usepackage[a4paper]{geometry}
\geometry{verbose, tmargin=2cm, bmargin=2cm, lmargin=2cm, rmargin=2cm, headsep=0.5cm, footskip=1cm}
\usepackage[hidelinks]{hyperref}  % PW causes a problem
%%    \newtheorem{APP.}{A.}[chapter]
\newtheorem{APP.}{}[chapter] % changed by PW

\begin{document}

\tableofcontents
  
\chapter{FFFFFFFFFFFFFFFFF}\label{chap: Brownian motion theory}
%%%    \justify % PW undefined control sequence
\pagenumbering{arabic}
%%%    \lhead{\textbf{\rightmark}} % PW undefined control sequence
\section{WWWWWW}\label{subsec: History of Brownian motion}
%%%    \justify % PW undefined control sequence
As pointed out in \ref{app.:law XXXXXX}...

\appendix  % added by PW
\chapter{Appendix} \label{app: Appendix ACCA}
\begin{APP.}\textbf{(Law XXXXXX)}\label{app.:law XXXXXX}
\noindent According to ...

\end{APP.}
    
\end{document}

I didn't like your Chapter 2 Appendix and changed that because an Appendix is not a chapter, although an appendix include one or more chapters. I also changed your \newtheorem specification. Now you get the reference style you are after.