Definition
(Cofinal)
.
C
⊆
κ
is
cofinal
(
=
unbounded) if
∀
λ
<
κ
,
∃
γ
∈
C
,
γ
≥
λ
. We can then define:
cf
κ
:
=
{
|
C
|
:
C
is cofinal
}
.