1. In topology, a subset of a topological space that contains all of its limit points; equivalently, a set whose complement is an open set.
In the real numbers with the standard topology, the interval [0,1] is a closed subset.
Nos números reais com a topologia padrão, o intervalo [0,1] é um subconjunto fechado.
2. A subset that is closed under the operations or relations defined on its parent set.
The set of even integers is a closed subset under addition.
O conjunto dos inteiros pares é um subconjunto fechado sob adição.