# Lebesgue dimension

A dimension defined by means of coverings (cf. Covering (of a set)). It is the most important dimension invariant $\dim X$ of a topological space $X$ and was discovered by H. Lebesgue . He stated the conjecture that $\dim I^n = n$ for the $n$-dimensional cube $I^n$. L.E.J. Brouwer  was the first to prove this, as well as the stronger identity: $\dim I^n = \text{Ind}\,I^n = n$. A precise definition of the invariant $\dim X$ (for the class of metric compacta) was given by P.S. Urysohn, who proved that for a space $X$ of this class $$\dim X = \text{ind}\,X = \text{Ind}\,X$$ (the Urysohn identity, see Dimension theory). This identity was extended to the class of all separable metric spaces by W. Hurewicz and L.A. Tumarkin in 1925.
For compacta $X$ the Lebesgue dimension is defined as the smallest integer $n$ having the property that for any $\epsilon > 0$ there is a finite open $\epsilon$-covering of $X$ that has multiplicity $\le n+1$; an $\epsilon$-covering of a metric space is a covering all elements of which have diameter $< \epsilon$, and the multiplicity of a finite covering of $X$ is the largest integer $k$ such that there is a point of $X$ contained in $k$ elements of the given covering. For an arbitrary normal (in particular, metrizable) space $X$ the Lebesgue dimension is the smallest integer $n$ such that for any finite open covering $\Omega$ of $X$ there is a (finite open) covering $\Lambda$ of multiplicity $n+1$ that refines it. A covering $\Lambda$ is said to be a refinement of a covering $\Omega$ if every element of $\Lambda$ is a subset of at least one element of $\Omega$.