Cardinality of Set of Induced Equivalence Classes of Surjection
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Theorem
Let $f: S \to T$ be a mapping.
Let $\RR_f \subseteq S \times S$ be the relation induced by $f$:
- $\tuple {s_1, s_2} \in \RR_f \iff \map f {s_1} = \map f {s_2}$
Let $f$ be a surjection.
Then there are $\card T$ different $\RR_f$-classes.
Proof
From the definition of a surjection:
- $\forall t \in T: \exists s \in S: \map f s = t$
Thus there are as many $\RR_f$-classes of $f$ as there are elements of $T$.
Hence the result.
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $4$: Mappings: Exercise $10 \ \text{(i) (b)}$