Linked from
The 12 pages that link to Semigroup, each with the reason it gives.
Group theoryCompared with: Semigroups isolate associativity from the other group axioms.
Identity elementNarrower topic: Adding an identity to a semigroup produces a monoid.
AssociativityBroader topic: Associativity is the defining law of a semigroup’s operation.
Binary operationBroader topic: Associativity adds the defining extra law beyond a bare operation.
MonoidNarrower topic: A monoid is precisely a semigroup equipped with an identity element.
MagmaBroader topic: Adding associativity to a magma produces a semigroup.