Defining new split environment with reduced spacing

Question

this is my first stackexchange question so apologies for any bad etiquette.

I'm writing a document in double spacing. I don't wants the gaps between the lines of a multi-line equation to be that large, though.

I know that I can make this change globally by using

\setlength{\jot}{<size>}

...but this also affects other spacings that I don't want to change, e.g. in an xymatrix or when using \gather

I also know that I can keep the changes local by using

\begingroup\setlength{\jot}{<size>}\begin{split}
...
\end{split}\endgroup

...but there is a very large number of split equations in my document and I'd prefer not to make it even messier by doing this.

I tried to define a new environment to effect this local change, like so

\newenvironment{Split}%
{\begingroup\setlength{\jot}{-3pt}\begin{split}}%
{\end{split}\endgroup}

But when I try to implement this in the document I get the following error message:

LaTeX Error: \begin{split} on input line xxx ended by \end{Split}.

I've had success in defining new environments before so I don't know what I'm doing wrong this time.

MWE:

\documentclass{article}

\usepackage{setspace}
\doublespacing

\usepackage{amsmath}

\newenvironment{Split}%
{\begingroup\setlength{\jot}{-3pt}\begin{split}}%
{\end{split}\endgroup}

\begin{document}

\begin{equation}
\begin{split}
y&=x+x\\
&=2x
\end{split}
\end{equation}

\begin{equation}
\begin{Split}
y&=x+x\\
&=2x
\end{Split}
\end{equation}

\end{document}

Answer 1

\documentclass{article}
\usepackage{setspace}
\usepackage{amsmath}
\usepackage{etoolbox}

\usepackage{lipsum} % just for the example
\usepackage[vmargin=2cm]{geometry} % just for the example

\doublespacing

\BeforeBeginEnvironment{split}{\linespread{1}\selectfont}{}{}

\begin{document}

\lipsum*[2]
\begin{equation}
\begin{split}
y&=x+x\\
&=2x
\end{split}
\end{equation}
\lipsum*[2]
\begin{align}
y&=x+x\\
&=2x
\end{align}
\lipsum*[2]
\begin{gather}
y=x+x\\
z=2x
\end{gather}
\lipsum*[2]

\end{document}

Answer 2

You can modify the \gather@split macro, calling \linespread{1}\selectfont before \spread@equation.

This won't affect other alignment environments.

\documentclass{article}
\usepackage{setspace}
\usepackage{amsmath}
\usepackage{etoolbox}

\usepackage{lipsum} % just for the example
\usepackage[vmargin=2cm]{geometry} % just for the example

\doublespacing

\makeatletter
\patchcmd{\gather@split}
  {\spread@equation}
  {\linespread{1}\selectfont\spread@equation}
  {}{}
\makeatother

\begin{document}

\lipsum*[2]
\begin{equation}
\begin{split}
y&=x+x\\
&=2x
\end{split}
\end{equation}
\lipsum*[2]
\begin{align}
y&=x+x\\
&=2x
\end{align}
\lipsum*[2]
\begin{gather}
y=x+x\\
z=2x
\end{gather}
\lipsum*[2]

\end{document}