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A topological space, also called an abstract topological space, is a set X together with a collection of open subsets T that satisfies the four conditions: 1. The empty set ...

A Z-action or N-action. A cascade and a single map X->X are essentially the same, but the term "cascade" is preferred by many Russian authors.

An abbreviation for the cokernel.

One of the Eilenberg-Steenrod axioms which states that, if X is a space with subspaces A and U such that the set closure of A is contained in the interior of U, then the ...

Given a subspace A of a space X and a map from A to a space Y, is it possible to extend that map to a map from X to Y?

A locally finite space is one for which every point of a given space has a neighborhood that meets only finitely many elements of any cover.

Convergents of the pi continued fractions are the simplest approximants to pi. The first few are given by 3, 22/7, 333/106, 355/113, 103993/33102, 104348/33215, ... (OEIS ...

If a polynomial P(x) is divided by (x-r), then the remainder is a constant given by P(r).

A refinement X of a cover Y is a cover such that every element x in X is a subset of an element y in Y.

An action with G=R^+.