# Unable to Use Array without Issue(s) bar turing red

In my Sage/CoCalc article (Sage has recently merged with CoCalc), I am unable to create an Array without a red issue bar popping up. Morover, I am unable to cross out element in my array. The issue(s) is `PackageArrayError; illegal pream-token (1): PackageArrayError; illegal pream-token (2)`

: ...... and so on.

The whole Document is below.

```
\documentclass{article}
% set font encoding for PDFLaTeX or XeLaTeX
\usepackage{ifxetex}
\ifxetex
\usepackage{fontspec}
\else
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\fi
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{ragged2e}
\usepackage{graphicx}
\usepackage{breqn}
\usepackage{fixltx2e}
\usepackage{hyperref}
\usepackage{titlesec}
\usepackage{amsopn}
\usepackage{array}
\newcommand{\dd}[1]{\mathrm{d}#1}
% used in maketitle
\title{Regarding Your Version of Natural Density to Rationals}
\author{Aiyappan Nair}
\begin{document}
\maketitle
Your version of asymptotic density to rationals differs from my own version. It is not a simplification but rather a change in definition. And although it is much more simple there are flaws that I would like to mention.
\subsection{Counter-Example}
As mentioned earlier, if sets $T_1,T_2,T_p$ are subsets of $\mathbb{Q}$, a density $D$ should have the following qualities.
(1) If $T_1=T_2$ then $D(T_1)=D(T_2)$
(2) If $T_1\subset{T_2}$ then $D(T_1)\le D(T_2)$
In your new version of the density
$$D=\lim_{r\to\infty}\frac{\left|T_1\cap V(r)\right|}{|V(r)|}$$
Where
$$V(r)=\left\{\left.\frac{m}{n}\right|1<m<r,1<n<r,\gcd(m,n)=1\right\}$$
and $m,n,r\in\mathbb{Z}$
If we set $T_1=\left\{\left.\frac{m}{2n+1}\right|m,n\in\mathbb{Z}\right\}$ and $T_2=\left\{\left.\frac{m}{4n+2}\right|m,n\in\mathbb{Z}\right\}$, I found that $D(T_1)>D(T_2)$ even though $T_1\subset T_2$. Which breaks requirment (2).
\subsection{Proof of Counter Example}
Imagine $V(r)$ as an array of values, restricted by $r$. The numerator is from listed as columns each labelled from $[1,r]$ and the denominator is listed as row labelled from $[1,r]$.
\begin{array}{|l|col1 col2 col3 col4 col5 col6 col7 col8}
& 1 & 2 & 3 & 4 & 5 & .. & r\\
\hline
1 & 1/1 & 2/1 & 3/1 & 4/1 & 5/1 & ..& r/1\\
2 & 1/2 & \st{2/2} & 3/2 & 4/2 & 5/2 & ..& r/2\\
3 & 1/3 & 2/3 & 3/3 & 4/3 & 5/3 & ..& r/3\\
4 & 1/4 & 2/4 & 3/4 & 4/4 & 5/4 & ..& r/4\\
5 & 1/5 & 2/5 & 3/5 & 4/5 & 5/5 & ..& r/5\\
..& .. & .. & .. & .. & .. & ..& .. \\
r & 1/r & 2/r & 3/r & 4/r & 5/r & .. & r/r\\
\end{array}
\end{document}
```

I have never had this problem with SageMath before it merged with CoCalc. Please help.

This has nothing to do with sagemath the free software. You should ask cocalc's customer support instead.