Let be an infinite Cardinal and be a Regular Cardinal with

Let be any family of sets of size such that

Then there is some such that and is a Delta System.

Proof

WLOG assume that there is some such that every has . Now we go by induction on . For , we get that is a System with root . Assume the result for , we prove it for . If some has many with , then remove it from everywhere and apply induction hypothesis. So assume that every has many with . Order . Then just build a chain of nonintersecting elements and use Cofinality.

Now the limit case. Assume the result for all . idk man does it even matter atp

Special case

Let and . Every uncountable family of finite sets contains an uncountable Delta System.