1.Previous
3.Next

definition adapts the presentation of a topological space in terms of neighborhood systems. A nonempty collection of subsets of is a ' (or a ') if it satisfies the following axioms:
# If then where is the diagonal on
# If and then
# If and then
# If then there is some such that , where denotes the composite of with itself. The composite of two subsets and of is defined by
# If then where

1.Previous
3.Next