\begin{align}
\label{eq:x}
x = \sign{a} \cdotp \sign{b} \cdotp \min(\lvert a \rvert,\lvert b \rvert)
\end{align}
Above gives me Undefined control sequence. Any ideas would be helpful.
[Tex/LaTex] Undefined control sequence in align
alignmath-operators
Related Solutions
Two variants:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat*}{2}
a &= b\\
c &= d\\
e &= f+g+h &{}+j &= w \\
& & x &= y\\
& & z &= 0
\end{alignat*}
\begin{align*}
a &= b\\
c &= d\\
e &= f+g+h\begin{aligned}[t]{}+j &= w \\
x &= y\\
z &= 0
\end{aligned}
\end{align*}
\end{document}
The {} in front of + is needed to get the spacing right, because TeX doesn't consider + a unary symbol.
Since no definition of \rulename is provided, it's hard to guess. However, I can reproduce the output with a “dummy” definition for it, so it possibly is just for markup (and good for possible redefinitions).
\documentclass{article}
\usepackage{amsmath}
\usepackage{proof}
\newcommand{\rulename}[1]{#1}
\begin{document}
\section{Propositional Logic}
\subsection{Introduction of $\wedge$}
\begin{equation*}
\infer[\rulename{\wedge_i}]
{\alpha \wedge \beta}
{\alpha && \beta}
\end{equation*}
(If you have derived $\alpha$ and $\beta$, then
you can conclude $\alpha \wedge \beta$.)
\end{document}
I have minimized the example; don't use align* as a surrogate for equation* when there is just one line.
Best Answer
Taking a hint from egreg, add
to your document preamble which would provide a
signoperator. However, since you're loadingamsmath,would be more robust.