Characteristic Function of Universe
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Theorem
Let $S$ be a set.
Let $\chi_S: S \to \set {0, 1}$ be its characteristic function (in itself).
Then:
- $\chi_S = f_1$
where $f_1: S \to \set {0, 1}$ is the constant mapping with value $1$.
Proof
From Characteristic Function Determined by 1-Fiber, $\chi_S$ is the mapping determined by:
- $\forall s \in S: \map {\chi_S} s = 1 \iff s \in S$
Thus:
- $\forall s \in S: \map {\chi_S} s = 1$
By definition of constant mapping:
- $\chi_S = f_1$
$\blacksquare$