基础拓扑学答案.pdf

n ex.10P.20 A , A , , A X X ∪Ai 1 2 n i 1 B ⊂X X ⇔ B ∩A A (i 1,2, ,n ) i i ⇒ ∀i ∈{1,2,,n}A −B ∩A B C ∩A B i i i X B C X B C ∩A A i i B ∩A A i i ⇐ B ∩A A (i 1,2, ,n ) ∀i ∈{1,2,,n} i i X B B ∩A B ∩A i i i i n n n n n n B (B ∩A ) (B ∩A ) ( B ) ∩( A ) ( B ) ∩X B ∪ ∪ ∪ ∪ ∪ ∪ i i i i i i i i 1 i 1 i 1 i 1 i 1 i 1 B X ex.13 P.20{x }(,τ ) n C N n N x →x ⇔ x x n n ⇐ ⇒ A {xn |xn ≠x }A C C N x ∈A A X x →x n n N