Proposition 3.2.2 of Engelking, Ryszard (1978). Dimension theory(PDF). North-Holland Mathematical Library. Vol. 19. Amsterdam-Oxford-New York: North-Holland. ISBN0-444-85176-3. MR0482697.
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Kuperberg, Krystyna, ed. (1995), Collected Works of Witold Hurewicz, American Mathematical Society, Collected works series, vol. 4, American Mathematical Society, p. xxiii, footnote 3, ISBN9780821800119, Lebesgue's discovery led later to the introduction by E. Čech of the covering dimension.