How to make a simple macro from this grouping?

Question

I'm frequently using the following group code to reduce vertical space between some equations (with the equation, align, gather and other maths environment) and the text above it:

\begingroup\abovedisplayskip=0pt
    some equations in there
\endgroup

But instead of typing this group code each time by hand, how could I create a simple macro in the preamble to do the same? I'm guessing that the macro should be used like the following, but can it be simpler?

\eqlift
     some equations in there
\eqdown

Here's a MWE code that shows the problem, which I solve with the code above (only after compilation):

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage{lmodern}
\usepackage{amsmath}

\begin{document}

Blabla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla blabla bla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}

\end{document}

And here's a preview of what it does:

I often get some too large vertical space for any class, any font size or other configuration I've tried in the past (see equation (3) in the preview picture). The only way I know to solve this, is to insert the group code I've shown above.

Answer 1

Normally TeX uses \abovedisplayskip above an equation, but if the last line of the previous paragraph is short then it uses \abovedisplayshortskip by default most classes set this to a shorter length and so the equation "tucks in" to the space left by the short line above.

The shortdisplayskip is used if the last line finishes 2em to the left of the equation, to make sure there is still a visual gap.

Your case (3) just happens to be very close to the cutoff, TeX is using the longer length the same as used in (4) but subjectively you want it to work like (1) and (2).

I offer two variants here.

Equations 5-8 show the setting when both lengths have the same value you then avoid the discontinuity as tex switches from one style to the other, but (8) ends up looking rather tight.

Equations 9-12 are set with the default settings as in your original, but the length of the last line of the third paragraph is made to look (to TeX) artificially shorter by inserting an arbitrary negative space. That causes TeX to insert the same space as for the first two paragraphs.

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage{lmodern}
\usepackage{amsmath}

\begin{document}

\section{Original}

Blabla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla blabla bla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}

\section{use abovedisplayshortskip everywhere}

{
\setlength\abovedisplayskip{\abovedisplayshortskip}


Blabla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla blabla bla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}

}

\section{abovedisplayshortskip in (3=11)}

Blabla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla bla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla :\hspace{-2cm}\mbox{}
\begin{equation}
    y(x) = a \, x^2.
\end{equation}
Blabla bla blabla bla blabla blabla bla bla blabla :
\begin{equation}
    y(x) = a \, x^2.
\end{equation}


\end{document}