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名詞

cactoid (複数形 cactoids)

1. A member of the cactus subfamily Cactoideae.
2. (topology) A locally connected continuum that is the closure of the union of an at most countable number of spheres and simple arcs, of which every simple closed contour is contained in one and only one of the spheres.
   • 1951, John William Theodore Youngs, The Representation Problem for Fréchet Surfaces, page 27:

     It is well known that a Peano space is a cactoid if and only if each of its true cyclic elements is a 2-sphere.