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Michał Masny
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What are the monoids in which every globally idempotent subsemigroup contains the identity element?

A semigroup is called globally idempotent when for any $x\in S$ there are $y,z\in S$ such that $x=yz$.

Is there a name for monoids whose every globally idempotent subsemigroup contains the identity element?

For example, the monoid $(\mathbb N,+)$ has this property, because if $S$ is a globally idempotent subsemigroup of $\mathbb N$, and $a$ is the smallest element of $S$, then $2a$ is the smallest element of $S+S$. Therefore $a=2a$, so $a=0$.

But there are many non-examples, even among groups.

Is anything known about such monoids?

Michał Masny
  • 1.4k
  • 1
  • 14
  • 26