Proof. Suppose S is the union of the left strongly simple subsemigroups Sα,
α ∈ Y . Then we have a ∈ (Sa2Sa] for every a ∈ S. In fact: Let a ∈ S, and let
a ∈ Sα for some α ∈ Y . Since Sα is a left strongly simple ordered semigroup
and a, a2 ∈ Sα, by Lemma 6, we have a ∈ (Sαa2Sαa]Sα
⊆ (Sa2Sa].
Let now L be a left ideal of S and a ∈ L. Since a, a2 ∈ S, we have
a ∈ (Sa2Sa] ⊆ (S(Sa4Sa2]Sa] = (S(Sa4Sa2)Sa] ⊆ ((Sa2)a2(Sa2Sa)].
Since a2 ∈ L, we have Sa2 ⊆ SL ⊆ L and Sa2Sa ⊆ SL ⊆ L. Thus we have
a ∈ (La2L] = (La2L]L, and L is intra-regular.
The converse statement: Suppose every left ideal of S is an intra-regular
subsemigroup of S. Then every left ideal of S is a semisimple subsemigroup
of S. Indeed: Let L be a left ideal of S and a ∈ L. Since L is intra-regular,
we have
a