How to use symbols such as \delta and \lambda

Question

I'm trying to use symbols such as \lambda and \delta but for some reason it's not compiling. I've reproduced the problem in the following code:

\documentclass[12pt,a4paper,oneside]{article}

\begin{document}

\begin{center}

M = < X,Y,S,ta, \delta int, \delta ext, \lambda >

\end{center}

\end{document}

I'm getting the following errors:

Missing $ inserted M = < X,Y,S,ta, \delta

Missing $ inserted I've inserted a begin-math/end-math symbol since I think

Missing } inserted I've inserted something that you may have forgotten.
Extra }, or forgotten \endgroup

Despite these errors, I can still view it in a PDF and it looks like the following:

So, just to clarify what I actually want:

1. To compile without the errors i.e. how do you make use of symbols like \delta etc?

2. How do I actually make it display like the following :

   

3. As depicted above, I want int and ext to be subscripts to the \delta symbols

Answer 1

You want a centered formula; so the correct way to do it is

\documentclass[12pt,a4paper,oneside]{article}

\begin{document}

\[
M = \langle X, Y, S, ta, \delta_{\mathrm{int}}, \delta_{\mathrm{ext}}, \lambda \rangle
\]

\end{document}

This is covered in any basic LaTeX manual, for example The not so short introduction to LaTeX