Idempotent and Regular Elements in (P(Wτ (X)); ·xi
)
For a term t let op(t) be the number of operation symbols occurring in t.
For each A ∈ P(Wτ (X)) we define the following sets:
A0
:= {a | a ∈ A and xi ∈ V ar({a})},
A00 := {a | a ∈ A and xi 6∈ V ar({a})},
Ar := {a | a ∈ A and op(a) = r}.
We consider the cases xi ∈ V ar(A) and xi 6∈ V ar(A). If xi 6∈ V ar(A),
then clearly A is idempotent. For the case that xi ∈ V ar(A) we show
xi ∈ A.
This is a consequence of the following more general lemma.