D
- DE-9IM (dimensionally extended nine-intersection model)
- A ISO standard model. It is a topological model used to describe the spatial relation of two geometries in two-dimensions (R2).
- Dense set
- A set is dense if it has nonempty intersection with every nonempty open set. Equivalently, a set is dense if its closure is the whole space.
- Derived set
- If X is a space and S is a subset of X, the derived set of S in X is the set of limit points of S in X.
- Developable space
- A toplogical space with a development.
- Development
- A countable collection of open covers of a toplogical space, such that for any closed set C and any point p in its complement there exists a cover in the collection such that every neighbourhood of p in the cover is disjoint from C.
- Diameter
- If (M, d) is a metric space and S is a subset of M, the diameter of S is the supremum of the distances d(x, y), where x and y range over S.
- Discrete metric
- The discrete metric on a set X is the function d : X × X → R such that for all x, y in X, d(x, x) = 0 and d(x, y) = 1 if x ≠ y. The discrete metric induces the discrete topology on X.
- Discrete space
- A space X is discrete if every subset of X is open. We say that X carries the discrete topology.
- Discrete topology
- See discrete space.
- Disjoint union topology
- See Coproduct topology.
- Dispersion point
- If X is a connected space with more than one point, then a point x of X is a dispersion point if the subspace X − {x} is hereditarily disconnected (its only connected components are the one-point sets).
- Distance
- See metric space.
- Dunce hat (topology)
Read more about this topic: Glossary Of Topology