Birkhoff's variety theorem (HSP theorem) says that a class of algebras of the same signature is a variety of algebras iff it is closed under homomorphic images (H), subalgebras (S) and small products (P), as proved in
The Birkhoff's subdirect representation theorem is proved in
On monoids (not on groupoids in the modern sense of that term):
On quantum logic:
On the Birkhoff duality between finite posets and finite distributive lattices
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