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

${\displaystyle \displaystyle \Delta _{\alpha <\delta }X_{\alpha },}$

is defined to be

${\displaystyle \displaystyle \{\beta <\delta \mid \beta \in \bigcap _{\alpha <\beta }X_{\alpha }\}.}$

That is, an ordinal ${\displaystyle \displaystyle \beta }$ is in the diagonal intersection ${\displaystyle \displaystyle \Delta _{\alpha <\delta }X_{\alpha ))$ if and only if it is contained in the first ${\displaystyle \displaystyle \beta }$ members of the sequence. This is the same as

${\displaystyle \displaystyle \bigcap _{\alpha <\delta }([0,\alpha ]\cup X_{\alpha }),}$

where the closed interval from 0 to ${\displaystyle \displaystyle \alpha }$ is used to avoid restricting the range of the intersection.

Relationship to the Nonstationary Ideal

For κ an uncountable regular cardinal, in the Boolean algebra P(κ)/I_NS where I_NS is the nonstationary ideal (the ideal dual to the club filter), the diagonal intersection of a κ-sized family of subsets of κ does not depend on the enumeration. That is to say, if one enumeration gives the diagonal intersection X₁ and another gives X₂, then there is a club C so that X₁ ∩ C = X₂ ∩ C.

A set Y is a lower bound of F in P(κ)/I_NS only when for any S ∈ F there is a club C so that Y ∩ C ⊆ S. The diagonal intersection ΔF of F plays the role of greatest lower bound of F, meaning that Y is a lower bound of F if and only if there is a club C so that Y ∩ C ⊆ ΔF.

This makes the algebra P(κ)/I_NS a κ⁺-complete Boolean algebra, when equipped with diagonal intersections.