• A is open in X. • A = Int A • A contains none of its boundary points. • Every point of A has a neighborhood contained in A.
• A is open in X. • A = Int A • A contains none of its boundary points. • Every point of A has a neighborhood contained in A.
Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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Transcribed Image Text:Proposition 8. Let X be a topological space and let ACX be any subset.
(a) A point is in Int A if and only if it has a neighborhood contained in A.
(b) A point is in Ext A if and only if it has a neighborhood contained in X\A.
(c) A point is in ÔA if and only if every neighhorhood of it contains both a point of A
and a point of X\A
(d) A point is in Ā if and only if every neighborhood of it contains a point of A.
(e)Ā = AU DA = Int A U ðA
(f) Int A and Ext A are open in X, while Ā and dA are closed in X.
(8) The following are equivalent: (Prove:)
• A is open in X.
• A = Int A
• A contains none of its boundary points.
• Every point of A has a neighborhood contained in A.
(h) The following are equivalent:
(Prove:)
• A is closed in X.
• A = Ā
• A contains all of its boundary points.
• Every point of X \A has a neighhorhood contained in X\A.
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