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?
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}
Another possible method is to use an invisible strut like \mathstrut.
\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage[intlimits]{amsmath}
\usepackage{physics}
\begin{document}
\[
\int_{x\mathstrut}^{\infty}\int_{b\mathstrut}^{\infty} x^2+y^2\,\dd x\,\dd y
\]
\end{document}
For other formatting options the »physics« package is quite helpful.
Place a \strut in each limit, so your example becomes
\[\int\limits_{x\strut}^{\infty}\int\limits_{b\strut}^{\infty} x^2+y^2 \mathrm{d}x \mathrm{d}y \]
A \strut is an invisible box of zero width but the same height and depth as (I think) an parenthesis.
\!(negative thinspace), and adding\,(positive thinspace) between the integrand and the variables of integration. – Mico Nov 18 '13 at 09:41\,dx\,dyis what you find in the TeXbook all the times. – Bakuriu Nov 18 '13 at 12:53