0. Set of symbols
1. Alphabet
2. Deduction system
3. Theory
4. Zermelo-Fraenkel set theory
5. Set
6. Binary cartesian set product
7. Binary relation
8. Map
9. Function
10. Real function
11. Euclidean real function
12. Basic real function
13. Basic rational function
14. Basic integer function
15. Basic natural number function
16. Basic boolean function
17. Indicator function
Indicator function operator
Formulation 0
Let $X$ be a D17: Finite set such that
(i) $\mathcal{P}(X)$ is the D80: Power set of $X$
(ii) $\{ 0, 1 \}^X$ is the D68: Set of maps from $X$ to $\{ 0, 1 \}$
The indicator function operator on $\mathcal{P}(X)$ is the D18: Map \begin{equation} \mathcal{P}(X) \to \{ 0, 1 \}^X, \quad E \mapsto I_E \end{equation}