How can I number subtheorems hierachically?

Question

I want to create two hierarchy levels of theorems. The goal is to have something like this:

Research Question 1: bla bla
Research Question 2: bla bla

And then in another section of the article, I would like to do something like this:

Research Question 1.1 bla bla
Research Question 1.2 bla bla

Research Question 2.1 bla bla
Research Question 2.2 bla bla

I used the following code:

\newtheorem{researchquestion}{Research Question}

\makeatletter
\newenvironment{subresearchquestion}[1]{%
  \def\subtheoremcounter{#1}%
  \refstepcounter{#1}%
  \protected@edef\theparentnumber{\csname the#1\endcsname}%
  \setcounter{parentnumber}{\value{#1}}%
  \setcounter{#1}{0}%
  \expandafter\def\csname the#1\endcsname{\theparentnumber.\Alph{#1}}%
  \ignorespaces
}{%
  \setcounter{\subtheoremcounter}{\value{parentnumber}}%
  \ignorespacesafterend
}
\makeatother
\newcounter{parentnumber}

However, the result is not exactly how it should look like: Research Question 1: bla bla Research Question 2: bla bla

And then in another section of the article, I would like to do something like this:

Research Question 3.1 bla bla
Research Question 3.2 bla bla

Research Question 4.1 bla bla
Research Question 4.2 bla bla

How can I achieve that the subtheorems are really subtheorems? I tried this approach: How to number subtheorems hierarchically? but the problem then is that the theorem numbering is influenced by the section numbering. I want this two things have independently.

Answer 1

See also Remove chapter number from theorem number

\documentclass{article}   

\newtheorem{theorem}{Theorem}[section]
\newtheorem{subtheorem}{Sub-Theorem}[theorem]

\renewcommand{\thetheorem}{\arabic{theorem}}
\renewcommand{\thesubtheorem}{\arabic{theorem}.\arabic{subtheorem}}

\begin{document}
\section{Introduction}

\begin{theorem} 
 Yada yada yada. 
\end{theorem}   

\begin{subtheorem}   
  Yada yada yada.
\end{subtheorem}

\begin{subtheorem}   
  Yada yada yada.
\end{subtheorem}

\begin{theorem} 
 Yada yada yada. 
\end{theorem}   

\begin{subtheorem}   
  Yada yada yada.
\end{subtheorem}

\begin{subtheorem}   
  Yada yada yada.
\end{subtheorem}

\end{document}