Definitions of T_0, T_1, etc. Topological Spaces



A space S is T0 when for all x,y, (cl {x} = cl {y} ==> x = y)
A space S is T1 when for all x, /\{ U | x in open U } = {x}

A space S is Hausdorff when for all distinct x,y,
some open disjoint U,V with x in U, y in V

A space S is regular when for all x, open U nhood x
some open V with x in V, cl V subset U
A space S is normal when for all closed K, open U
K subset U ==> some open V with K subset V, cl V subset U

Exercise. Normal T1 ==> Regular T0 ==> Hausdorff ==> T1 ==> T0.
Exercise. Give an example of an irregular normal space.

Exercise. Demostrate that not only are T2, T3 and T4 not needed,
but also that their use is most confusing and misleading,
leaving one to be lost in some regular abnormal space.
Exercise. Don't ask about T_(1/2) and T_-1.

.



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