Rough sets induced by quasiorders appear in several constructions usingbinary relations in computer science. In this paper, a structuralcharacterisation of rough sets induced by quasiorders is given. These roughsets form Nelson algebras defined on algebraic lattices. We prove that anyNelson algebra can be represented as a subalgebra of an algebra defined onrough sets induced by a suitable quasiorder. We also show that Monteiro spaces,rough sets induced by quasiorders and Nelson algebras defined on $\rmT_0$-spaces that are Alexandrov topologies can be considered as equivalentstructures, because they determine each other up to isomorphism.
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