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$\mbox{${\mathbb{Q}}$}$

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$f_1:3{\mbox{${\mathbb{Z}}$}}\ni x \mapsto 4x \in 6{\mbox{${\mathbb{Z}}$}}$ .
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$f_3: 2 {\mbox{${\mathbb{Z}}$}}\ni x \mapsto 4 x \in 8 {\mbox{${\mathbb{Z}}$}}$ .
$f_4: 2 {\mbox{${\mathbb{Z}}$}}\ni x \mapsto x^3 \in 8 {\mbox{${\mathbb{Z}}$}}$ .

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