The least order-type of an unbounded subset of α. Regular cardinals (cf(κ)=κ) vs singular cardinals (cf(κ)<κ) is a central dichotomy of infinite combinatorics.
The least order-type of an unbounded subset of α. Regular cardinals (cf(κ)=κ) vs singular cardinals (cf(κ)<κ) is a central dichotomy of infinite combinatorics.