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<p>every <a href="page.php?w=abelian_group">abelian group</a> a commutative monoid.<br/>
* Any <a href="page.php?w=semigroup">semigroup</a>  may be turned into a monoid simply by adjoining an element  not in  and defining  for all . This conversion of any semigroup to the monoid is done by the <a href="page.php?w=free_functor">free functor</a> between the category of semigroups and the category of monoids.<br/>
** Thus, an idempotent monoid (sometimes known as find-first) may be formed by adjoining an identity element  to the <a href="page.php?w=left_zero_semigroup">left zero semigroup</a></p><p>
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