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::: {.Definition #magma title="magma"}
A magma is a set M with an operation ⋅ that sends any two elements a, b ∈ M to another element, a⋅b∈M. The symbol ⋅ is a general placeholder for a properly defined operation. This requirement that for all a, b in M, the result of the operation a⋅b also be in M, is known as the magma or closure property.
:::
::: {.Definition #semigroup title="semigroup"}
To understand semigroup, you need to know magma...
:::: {#incmagma include="magma"}
:::::
... Now you know magma, let's see the definition of semigroup.
(S, ⋅) is a semigroup if it is an associative magma.
:::
::: {.Definition #monoid title="monoid"}
To understand monoid, you need to know semigroup...
:::: {#incsemigroup include="semigroup"}
:::::
... Now you know semigroup, let's see the definition of monoid.
A monoid is a semigroup with an identity element.
:::
::: {.Definition #group title="group"}
To understand group, you need to know monoid...
:::: {#incmonoid include="monoid"}
:::::
... Now you know monoid, let's see the definition of group.
(S, ⋅) is a group is it is a monoid such that every element has an unique inverse.
:::