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Math 436 Introduction to Topology Spring 2014 Follow-up on derived sets Class on February 4 studied how intersection, union, and complement interact with interior, closure, frontier, exterior, and the derived set in a topological space ๐. Here is a summary. Interior Intersection (๐ด โฉ ๐ต)โฆ = (๐ดโฆ ) โฉ (๐ต โฆ ) for all sets ๐ด and ๐ต. Union (๐ดโฆ ) โช (๐ต โฆ ) always is a subset of (๐ด โช ๐ต)โฆ and sometimes is a proper subset. Complement (๐ โงต ๐ด)โฆ always is a subset of ๐ โงต (๐ดโฆ ) and sometimes is a proper subset. Closure Intersection Cl(๐ด โฉ ๐ต) always is a subset of (Cl ๐ด) โฉ (Cl ๐ต) and sometimes is a proper subset. Union Cl(๐ด โช ๐ต) = (Cl ๐ด) โช (Cl ๐ต) for all sets ๐ด and ๐ต. Complement ๐ โงต (Cl ๐ด) always is a subset of Cl(๐ โงต ๐ด) and sometimes is a proper subset. Frontier Intersection Fr(๐ด โฉ ๐ต) and (Fr ๐ด) โฉ (Fr ๐ต) have no general relationship to each other. Union Fr(๐ด โช ๐ต) always is a subset of (Fr ๐ด) โช (Fr ๐ต) and sometimes is a proper subset. Complement Fr(๐ โงต๐ด) and ๐ โงต(Fr ๐ด) are complements of each other, since Fr(๐ โงต๐ด) = Fr ๐ด. Exterior Intersection (Ext ๐ด) โฉ (Ext ๐ต) always is a subset of Ext(๐ด โฉ ๐ต) and sometimes is a proper subset. Union Ext(๐ด โช ๐ต) always is a subset of (Ext ๐ด) โช (Ext ๐ต) and sometimes is a proper subset. Complement Ext(๐ โงต ๐ด) always is a subset of ๐ โงต (Ext ๐ด) and sometimes is a proper subset. Derived set Intersection (๐ด โฉ ๐ต)โฒ always is a subset of (๐ดโฒ ) โฉ (๐ต โฒ ) and sometimes is a proper subset. Union (๐ด โช ๐ต)โฒ = (๐ดโฒ ) โช (๐ต โฒ ) for all sets ๐ด and ๐ต. Complement (๐ โงต ๐ด)โฒ and ๐ โงต (๐ดโฒ ) have no general relationship to each other. February 6, 2014 Page 1 of 1 Dr. Boas