Lyx doesn't compile a kth root

Question

A LyX document I wrote won't compile after I added in a mathematical formula the command root (in fact, \root[k]{\alpha}). I would have written you the code, but the document is long (over than 20 pages), so it would be too much. Any ideas on how to solve it?

Thanks in advance.

Edit:

The specific line is:

Let $\alpha\in\mathbb{R}$. Then 
\[
\sqrt[k]{\alpha}
\]

And the error message is:

"A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)"

Edit 2:

Here is the problematic part, I created a new document and this one doesn't compile on LyX.]

%% LyX 2.0.6 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{geometry}
\geometry{verbose,tmargin=2.54cm,bmargin=2.54cm,lmargin=3.18cm,rmargin=3.18cm}
\usepackage{mathrsfs}
\usepackage{amsthm}
\usepackage{amsmath}
\usepackage{amssymb}

\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
\theoremstyle{plain}
\newtheorem{thm}{\protect\theoremname}[section]
\theoremstyle{definition}
\newtheorem{defn}[thm]{\protect\definitionname}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
\usepackage{bbold}
\DeclareSymbolFont{bbold}{U}{bbold}{m}{n}
\DeclareSymbolFontAlphabet{\mathbbold}{bbold}
\setcounter{section}{-1}
\renewcommand{\raise}[2]{#1^#2}

\makeatother

\usepackage{babel}
\providecommand{\definitionname}{Definition}
\providecommand{\theoremname}{Theorem}

\begin{document}

\title{Try}

\maketitle
\global\long\def\layer#1#2{\overset{\left[#2\right]}{}#1}
\global\long\def\ELT#1#2{\mathscr{R}\left(#2,#1\right)}

\begin{defn}
Let $R=\ELT{\mathbb{R}}{\mathbb{C}}$, and take $\alpha=\layer a{\ell}\in R$
and $k\in\mathbb{N}$. Then $\sqrt[k]{\alpha}$.\end{defn}

\end{document}

Answer 1

You have redefined the the internal command \raise used for moving boxes up and down. Such boxes typically contain characters to typeset. This is used in the optional argument of the \sqrt command to get the symbol at an appropriate height, and in many other places in LaTeX. In fact, I am surprised that nothing else in your document triggered the error. Removing your redefition of \raise, gives a file that compiles without errors.

The fact that you had to use \renewcommand rather than \newcommand should have lead you to check what command you were redefining. One first way to find the current definition of a command is to write \show before it. For a standard file \show\raise produces

> \raise=\raise.

indicating that it is a primitive. In comparison \show\sqrt gives

> \sqrt=macro:
->\protect \sqrt  .

demonstrating that \sqrt is a macro, whereas \show\myundefinedcommand gives

> \myundefinedcommand=undefined.

Other methods for unravelling command definitions are discussed in the questions

• Display source for a command and
• Equivalent of \show to display the LaTeX code in the document.