# Today I Learned

Some of the things I've learned every day since Oct 10, 2016

## 3: Monoids

October 13, 2016

Posted by on A **monoid** is a group-like structure. It can be regarded as a *semigroup with an identity element*, or as a *group without invertibility*.

In category theory, a monoid can also be regarded as a category with a single object, where the morphisms of the category correspond to the elements of the monoid, and composition of morphisms corresponds to the operation of the monoid.

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