Diagonal intersection

Diagonal intersection is a term used in mathematics, especially in set theory.

If ${\displaystyle \displaystyle \delta }$ is an ordinal number and ${\displaystyle \displaystyle \langle X_{\alpha }\mid \alpha <\delta \rangle }$ is a sequence of subsets of ${\displaystyle \displaystyle \delta }$, then the diagonal intersection, denoted by

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