# “Undefined control sequence. [\end{flalign*}]” error I can't seem to fix

I actually get the error twice on the same line. Can't figure out what's causing it. My code:

\documentclass{article}
\usepackage[left=1in, top=1in, bottom=1in]{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}

\begin{flalign*}
\textbf{14c)} \quad & \text{Let } V(x,y) \text{ be "} x \text{ has visited } y \text{",} &\\
& \text{where } x \in \{ \text{students in this class} \} \text{ and } y \in \{ \text{places} \} &\\
& \exists x (V(x, \text{Alaska}) \land \neg V(x, \text{Hawaii})) &\\
\textbf{d)}   \quad & \text{Let } L(x,y) \text{ be "} x \text{ has learned at least } y \text{ programming languages",} &\\
& \text{where } x \in \{ \text{students in this class} \} \text{ and } y \in \R &\\
& \forall x (L(x, 1)) &\\
\textbf{e)}   \quad & \text{Let } T(x,y) \text{ be "} x \text{ has taken every course offered by } y \text{",} &\\
& \text{where } x \in \{ \text{students in this class} \} \text{ and } y \in \{ \text{departments in this school} \} &\\
& \exists x,y (T(x,y))
\end{flalign*}
\end{document}


The error: 2.tex:80: Undefined control sequence. [\end{flalign*}]

• \R is not defined. If you remove that, the code runs fine for me. – Troy Jan 19 '17 at 3:19

## 2 Answers

It is probably what you needed (\R is now defined as blackboard R):

\documentclass{article}
\usepackage[left=1in, top=1in, bottom=1in]{geometry}
\usepackage{amsmath}
\usepackage{amssymb}

\newcommand\R{\mathbb{R}}

\begin{document}

\begin{flalign*}
\textbf{14c)} \quad & \text{Let } V(x,y) \text{ be "} x \text{ has visited } y \text{",} &\\
& \text{where } x \in \{ \text{students in this class} \} \text{ and } y \in \{ \text{places} \} &\\
& \exists x (V(x, \text{Alaska}) \land \neg V(x, \text{Hawaii})) &\\
\textbf{d)}   \quad & \text{Let } L(x,y) \text{ be "} x \text{ has learned at least } y \text{ programming languages",} &\\
& \text{where } x \in \{ \text{students in this class} \} \text{ and } y \in \R &\\
& \forall x (L(x, 1)) &\\
\textbf{e)}   \quad & \text{Let } T(x,y) \text{ be "} x \text{ has taken every course offered by } y \text{",} &\\
& \text{where } x \in \{ \text{students in this class} \} \text{ and } y \in \{ \text{departments in this school} \} &\\
& \exists x,y (T(x,y))
\end{flalign*}
\end{document}


The error message, as reported on the log file, is

! Undefined control sequence.
<argument> ...is class} \} \text { and } y \in \R
&\\ & \forall x (L(x, 1)) ...
l.17 \end{flalign*}


which makes pretty clear what's the problem: the command \R has no definition by default and it should be defined, possibly with

\newcommand{\R}{\mathbb{R}}


or

\newcommand{\R}{\mathbf{R}}


according to your stylistic preferences.

You may want to consider using a list, rather than that cumbersome flalign environment.

\documentclass{article}
\usepackage[
left=1in,
top=1in,
bottom=1in,
showframe,
]{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{enumitem}

\newcommand{\R}{\mathbb{R}}

\begin{document}

\begin{enumerate}[
leftmargin=3.5em,
labelwidth=2.5em,
labelsep=1em,align=right,
]

\item[\bfseries 14c)]
Let $V(x,y)$ be $x$ has visited $y$'', where
$x \in \{ \text{students in this class} \}$
and $y \in \{ \text{places} \}$ \\*
$\exists x (V(x, \text{Alaska}) \land \neg V(x, \text{Hawaii}))$

\item[\bfseries d)]
Let $L(x,y)$ $x$ has learned at least $y$ programming languages'',
where $x \in \{ \text{students in this class} \}$ and $y \in \R$, \\*
$\forall x (L(x, 1))$

\item[\bfseries e)]
Let $T(x,y)$ be $x$ has taken every course offered by $y$'',
where $x \in \{ \text{students in this class} \}$ and
$y \in \{ \text{departments in this school} \}$ \\*
$\exists x,y (T(x,y))$

\end{enumerate}

\end{document}


The showframe option is responsible for the thin lines around the text block, comment it for the production version.