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A locally connected continuum $C$ that is the closure of the sum of at most a countable number of spheres $S_i$ and simple arcs $D_i$ located in Euclidean space $\mathbf{E}^3$, and such that for each closed contour $L \subset C$ there exists exactly one sphere $S_i$ containing it. Cactoids, and they alone, are monotone images of the $2$-dimensional sphere $S^2$; also, every cactoid is a monotone open image of $S^2$.

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Cactoid. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by B.A. Efimov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article