ThmDex – An index of mathematical definitions, results, and conjectures.

ThmDex – An index of mathematical definitions, results, and conjectures.

Result R2370 on D41: Indicator function

Real arithmetic expression for cardinality of countable set

Formulation 0

Let $X$ be a D16: Countable set.

Then \begin{equation} |X| = \sum_{x \in X} I_X(x) \end{equation}