Lv 4
ALSON asked in 科學數學 · 1 decade ago


1. A為閉集⇔∀{Xn}為A上的序列,且Xn→X∈M⇒X∈A

2. A為cpt⇒A為cld



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2 Answers

  • ?
    Lv 7
    1 decade ago
    Favorite Answer

    請問一下 你的A是那一種空間?

    2008-01-13 00:48:09 補充:

    1.(pf)(=>)Suppose otherwise,that is ,there exist a sequence {x_n} in A

    x_n->x,but x不屬於A,so x€M\A

    Since x_n->x,for any ε>0,choose N, such that for n>=N,we have

    ||x_n-x||<ε,x_n in A,so the open ball B(x,ε) does not contained in M\A

    =>M\A is not open

    =>A is not closed-----------><------------

    so we done

    (<=)Let x be an accumnlation point of A,we must show that x€A

    Now for any positive integer n,choose x_n in A with x_n≠x

    such that ||x_n-x||<1/n

    We claim that x_n->x as n->∞:

    Given any ε>0, choose 1/N<ε, then for n>=N


    so x_n->x as n->∞

    By hypothesie,x€A

    and so A is closed

    2.Suppose A is compact ,first prove that A is bounded,for p€A.B(p,k) k=1,2,.... form an open covering of A,By compactness of A,there exist M with A is covering by ∪_(i=1~M)B(p,i)

    Hence A is bounded

    Next we will show that A is closed,Suppose A is not closed,that is there is an accumnlation point y of A but y is not belong to A

    Let r_x=||x-y||,x€A,hence r_x>0,then {B(x,r_x),x€A} form an open covering of A,By compastness of A again,there is N such that ∪(i=1~N)B(xi,r_i) also covers A

    Letting r=min{r_i:i=1,...,N} then for x€B(y,r),||x-y||<r<=r_i



    =>x is not belong to B(x_i,r_i) for all i=1,...,N

    =>B(y,r)∩A is empty

    this is contracditis y is an accmulation point of A

    so A is closed

    2008-01-13 23:30:29 補充:



    Lv 4
    1 decade ago


    The topology of euclidean space

    Compact and Connected sets


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