ThmDex – An index of mathematical definitions, results, and conjectures.
Result R4888 on D15: Set cardinality
Isotonicity of set cardinality
Formulation 0
Let $X$ and $Y$ each be a D11: Set such that
(i) $X \subseteq Y$ is a D78: Subset of $Y$
Then \begin{equation} |X| \leq |Y| \end{equation}