Is every free monoid a pure monoid and vice versa?

Question

A pure monoid is a monoid where only the identity has an inverse. Is every free monoid pure, and conversely?

score 3 · Accepted Answer · answered Dec 02 '19 at 18:34

Yes, free monoids are pure. By definition a free monoid on a set is the monoid of words consisting of elements from that set, and there is no cancellation, so no inverses, apart from the empty word.

The non-negative rationals under addition is not a free monoid, but it is pure.

score 2 · Answer 2 · answered Dec 03 '19 at 08:12

2

Minimal example. The multiplicative monoid $\{1,0\}$ is pure, but not free.

answered Dec 03 '19 at 08:12

J.-E. Pin

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Is every free monoid a pure monoid and vice versa?

2 Answers2