How do I write this equation?

Question

I am trying to write some Greek text in PDF, but the first word of the equation is incomprehensible to me.

Answer 1

Use the following code:

\documentclass{article}
\usepackage{mathtools}% <-- for aligned environment
\begin{document}
[
    \begin{aligned}
        \mathcal{T}&={(x,\xi) \mid 0\leq\xi\leq x\leq 1} \
        \mathcal{T}_1&=\mathcal{T}\times{t\geq 0} \
        \mathcal{S}&={(x,\xi) \mid 0\leq x\leq\xi\leq 1} \
        \mathcal{S}_1&=\mathcal{S}\times{t\geq 0}
    \end{aligned}
]
\end{document}

which gives

EDIT

Alternatively, align* environment will also do. (Thanks to sigur)

\documentclass{article}
\usepackage{mathtools}% <-- for align environment
\begin{document}
    \begin{align}
        \mathcal{T}&={(x,\xi) \mid 0\leq\xi\leq x\leq 1} \
        \mathcal{T}_1&=\mathcal{T}\times{t\geq 0} \
        \mathcal{S}&={(x,\xi) \mid 0\leq x\leq\xi\leq 1} \
        \mathcal{S}_1&=\mathcal{S}\times{t\geq 0}
    \end{align}
\end{document}