How to `ignore' space demands in math mode

Question

Sorry for the title, I couldn't come up with a better one. I need to typeset an integral like this.

\int\limits_{f(x) < g(x)} \mathrm{d}x\,F(x)

The point is that the condition for the domain of integration is very lengthy. LaTeX will center this under the integral and put a huge space between the integral symbol and the integrand/differential symbol. I would like to delete this space, ideally to the left and to the right, because in many cases there will be nothing that could collide with the domain of integration.

So I came up with this:

\newcommand{\nint}[1]{\int\limits_{\begin{minipage}[t]{1.7em}\centering \mbox{$ \scriptstyle #1 $}\end{minipage}}}

The result is not bad (but as you can see, \centering has no effect. Not that I expected this to work, but I thought it was worth a try):

\[   C\,\vert E_0 \vert\,\nint{V(x) < E_0} \dd^d x\,\vert V_-(x) \vert^{d/2}  \]

Nevertheless, I am not completely pleased. I would like to use some space from the left, too. My dream would be, since I will have conditions a < b, to center a symbol (<) under the integral. But I would also be happy with a centering of the whole thing under the integral.

Any suggestions, anyone?

Answer 1

You can use \smashoperator (with care, abusing it could lead to bad output):

\documentclass{article}
\usepackage{mathtools}

\newcommand{\diff}[1][]{%
  \mathrm{d}%
  \if\relax\detokenize{#1}\relax\else^{#1}\fi
}

\begin{document}

\[
C\lvert E_0\rvert
\smashoperator{\int\limits_{V(x)<E_0}}
  \diff[d]x\,\lvert V_{-}(x)\rvert^{d/2}
\]

\end{document}