The small and large inductive dimensions, together with the Lebesgue covering dimension are the three main notions of dimension for topological spaces. They coincide in some, but not all, cases.
The small inductive dimension (alias Menger–Urysohn dimension) of a regular topological space is defined as follows: and () if for any and any neighborhood of there is an open subset such that , , and .
The large inductive dimension (alias Brouwer–Čech dimension) of a normal topological space is defined as follows: and () if for any closed subset and any open subset such that there is an open subset such that and .
The subspace theorem: for every subspace of a regular space we have . (Theorem 1.1.2 in Engelking95.)
notion of dimension
Ryszard Engelking, Dimension Theory, Mathematical Library 19, North-Holland Publishing/Polish Scientific Publishers 1978 (pdf)
Ryszard Engelking, Theory of Dimensions – Finite and Infinite, Sigma Series in Pure Mathematics 10, Helderman 1995 (pdf)
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