# Runaway argument

The following code:

\begin{align*} \lim_{u\rightarrow\infty}\frac{\psi(u)}{\overline{F_I}(u)}&=\left(\frac{\rho}{1+\rho}\right)\sum_{n=0}^{\infty}n(1+\rho)^{-n}\\
&=\left(\frac{\rho}{1+\rho}\right)\sum_{n=0}^{\infty}\sum_{m=0}^{n}(1+\rho)^{-n}\\
&=\left(\frac{\rho}{1+\rho}\right)\sum_{m=0}^{\infty}\sum_{n=m}^{\infty}(1+\rho)^{-n}\\
&=\sum_{m=0}^{\infty}(1+\rho)^{-m}=\frac{(1+\rho)^{-1}}{1-(1+\rho)^{-1}}=\rho^{-1}.\qed \end{align*}


produces following error:

"! Paragraph ended before \align* was complete.
\par "


What seems to be the problem?

Edit:

Actually, the culprit was the following alignment above the original alignment

\documentclass[a4paper, twoside, ngerman, openright, 12pt]{report}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\begin{document}
\begin{align*}
\frac{\psi(u)}{\overline{F_I}(u)}&=\left(\frac{\rho}{1+\rho}\right)\sum_{n=1}^{\‌​infty}(\frac{\lambda\mu}{c})^{n}\frac{\overline{F_I^{n }}(u)}{\overline{F_I}(u)}\\
&\leq K\left(\frac{\rho}{1+\rho}\right)\sum_{n=1}^{\infty}\left(\frac{1+\rho\right)^{-n}(1+\epsilon)^{n}<\infty.
\end{align*}
\end{document}

• The correct command for setting the QED symbol in a final alignment environment is \qedhere (assuming you're using amsthm). – egreg Apr 26 '12 at 21:14
• Your code compiles fine with the appropriate packages (amsmath and amsthm), there must be something in your preamble that is causing the problem. – Peter Grill Apr 26 '12 at 21:15
• While code snippets are useful in explanations, it is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. In this case I suspect that you'll solve your own problem by the time you compose a MWE. – Peter Grill Apr 26 '12 at 21:17
• The error suggests that you are leaving some blank line(s) inside an align* environment; if that's so, simply remove those blank lines; otherwise, as Peter Grill suggested, add a MWE. – Gonzalo Medina Apr 26 '12 at 21:20
• There was an alignment over the original alignment, which seems to create the problem: \documentclass[a4paper, twoside, ngerman, openright, 12pt]{report} \usepackage[T1]{fontenc} \usepackage[ngerman]{babel} \begin{document} – Chris Apr 26 '12 at 21:26

There's an incomplete \frac command at the end of the second line of the align*, here:

\left(\frac{1+\rho\right)


perhaps you meant

\left(\frac{\rho}{1+\rho}\right)


Also you end align* with align (no star), but perhaps that was only a typo in the MWE? Your equation in a complete example:

\documentclass[a4paper, twoside, ngerman, openright, 12pt]{report}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{amsmath}

\begin{document}

\begin{align*}
\frac{\psi(u)}{\overline{F_I}(u)} &= \left(\frac{\rho}{1 + \rho}\right)\sum_{n=1}^{\infty}(\frac{\lambda\mu}{c})^{n}\frac{\overline{F_I^{n }}(u)}{\overline{F_I}(u)} \\
&\leq K\left(\frac{\rho}{1+\rho}\right)\sum_{n=1}^{\infty}\left(\frac{\rho}{1+\rho}\right)^{-n}(1+\epsilon)^{n}<\infty.
\end{align*}

\end{document} • You are right, the \frac is the problem, it should not have been there at all: – Chris Apr 26 '12 at 22:02