\documentclass[a4paper,11pt]{article}
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\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{amsmath}
\usepackage{stackrel}
\usepackage{geometry}

\geometry{width=126mm}



\begin{document}

\noindent
$X \stackrel{\text{def}}{=} \{x\in \mathbb{N}\colon x^2 < 1000 \}$\\[2mm]
$A \stackrel{\tau}{\longrightarrow} B$\\[2mm]
$A_i \stackrel[i \ne j]{}{=} A_j$\\[2mm]
$X \stackrel[a]{b}{\approx} Y$

\end{document}