[Tex/LaTex] How to align limits of double integrals

Question

I'd like to have the limits of two integrals vertically aligned.

In this example, the x and b should be positioned on the same line.

\[\int\limits_x^{\infty}\int\limits_b^{\infty} x^2+y^2 \mathrm{d}x \mathrm{d}y \]

Has anybody an idea how to achieve this?

Answer 1

One way is to use \vphantom{b} with x:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[\int\limits_{\vphantom{b}x}^{\infty}\int\limits_b^{\infty} x^2+y^2 \mathrm{d}x \mathrm{d}y \]
\end{document}

With some beautifications as per the comments of Mico:

\documentclass{article}
\usepackage{amsmath}
\newcommand*\diff{\mathop{}\!\mathrm{d}}
\begin{document}
\[\int\limits_{\vphantom{b}x}^{\infty}\!\!\int\limits_b^{\infty} x^2+y^2 \,\diff x \, \diff y \]
\end{document}