Irreducible mapping

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A continuous mapping of a topological space $X$ onto a topological space $Y$ such that the image of every closed set in $X$, other than $X$ itself, is different from $Y$. If $f : X \rightarrow Y$ is a continuous mapping, $f(X) = Y$, and if all inverse images of points under $f$ are compact, then there exists a closed subspace $X_1$ in $X$ such that $f(X_1) = Y$ and such that the restriction of $f$ to $X_1$ is an irreducible mapping. The combination of the requirements on a mapping of being irreducible and being closed has an outstanding effect: Spaces linked by such mappings do not differ in a number of important cardinal characteristics; in particular, they have the same Suslin number and $\pi$-weight. But the main value of closed irreducible mappings lies in the central role they play in the theory of absolutes.


[1] A.V. Arkhangel'skii, V.I. Ponomarev, "Fundamentals of general topology: problems and exercises" , Reidel (1984) (Translated from Russian)


[a1] J.R. Porter, R.G. Woods, "Extensions and absolutes of Hausdorff spaces" , Springer (1988)
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Irreducible mapping. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by A.V. Arkhangel'skii (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article