基本介紹
![冪集公理](/img/2/1ac/wZwpmL2QTO0gzNxMDM3UzM1UTM1QDN5MjM5ADMwAjMwUzLzAzL2UzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
這個公理說明:“對於任何的x,存在著一個集合y,使y的元素是而且只會是x 的子集。”換句話說:給定任何集合x,有著一個集合P(x),使得給定任何集合z,z 是P(x)的成員,若且唯若z 是x 的子集。通過外延公理可知,這個集合是唯一的。我們可以稱集合P(x)為x的冪集。所以這個公理的本質是:所有集合都有一個冪集。冪集公理一般被認為是無可爭議的,它或它的等價者出現在所有可替代的集合論的公理化中。冪集公理允許定義兩個集合X和Y的笛卡兒積:
![冪集公理](/img/3/6b5/wZwpmL2MjNzIzN2QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzIzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
笛卡兒積為一個集合是因為
![冪集公理](/img/7/ffa/wZwpmLzQTMzUDN5gzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4czL1EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
可以遞歸地定義集合的任何有限的蒐集的笛卡兒積:
![冪集公理](/img/8/0f5/wZwpmL3QjMxkDNwEDM3UzM1UTM1QDN5MjM5ADMwAjMwUzLxAzL1EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
注意,在不包含冪集公理的克里普克—布萊特集合論中,笛卡兒積的存在性是可以證明的 。
相關性質定理
由外延公理知,B是唯一的,並稱B是A的冪集。因此有如下定義。
![冪集公理](/img/6/276/wZwpmL2UTN0ATM5MTO2UzM1UTM1QDN5MjM5ADMwAjMwUzLzkzL1UzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/c/c8f/wZwpmL3gTMycjM4QTOwADN0UTMyITNykTO0EDMwAjMwUzL0kzL3MzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![冪集公理](/img/5/8fe/wZwpmLwIzN1gDO1ADO3EDN0UTMyITNykTO0EDMwAjMwUzLwgzLxAzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
定義1集合B是集合A的一個冪集,若且唯若 的冪集 記作 ,便有
![冪集公理](/img/b/3e8/wZwpmLzMTM4EDN3QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzL0czLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
關於冪集的討論,從有限集合的冪集開始。為此,先定義有限集合。
定義2對於一個集合A,如果存在自然數n,使得A恰有n 個元,則稱A 是有限的。
![冪集公理](/img/2/cbb/wZwpmLzczMzYzMyYDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2gzLzczLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/3/467/wZwpmL0cDNyMDM0IDN3UzM1UTM1QDN5MjM5ADMwAjMwUzLyQzL2MzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
定理1若A是一個有限集合,且 令 則C的所有子集的個數恰是A 的所有子集的個數的二倍。
證明:因為對於A的每個子集,可加入B和不加人B這個元,這樣便得到C的所有子集,所以C的子集個數恰是A的子集個數的兩倍。
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/6/781/wZwpmLxgjMyYDMyQTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0kzL4AzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
定理2對於自然數n,如果集合A恰有個n元,則 恰有 個元。
![冪集公理](/img/b/933/wZwpmLyIjNxcTM4kTNwMDN0UTMyITNykTO0EDMwAjMwUzL5UzL0QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/f/077/wZwpmLyEzM4gzM4IzM3UzM1UTM1QDN5MjM5ADMwAjMwUzLyMzLzUzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/6/781/wZwpmLxgjMyYDMyQTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0kzL4AzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/6/3f1/wZwpmL1cjMxQDM3IjN2UzM1UTM1QDN5MjM5ADMwAjMwUzLyYzL1QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![冪集公理](/img/0/2e3/wZwpmLxIDN0IjN5YzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL2czL1YzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
證明:用:的歸納原理加以證明。對於n,設是命題:對任意集合A,若A有n個元,則有個元。本定理證明化為:也即要證:是個歸納集,因此,
![冪集公理](/img/8/314/wZwpmL2gzN4UDNxEDN3UzM1UTM1QDN5MjM5ADMwAjMwUzLxQzL3gzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/3/253/wZwpmLwEjMzMTN1UjN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1YzLyEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![冪集公理](/img/1/72a/wZwpmL3gTM2cjN1ADN3UzM1UTM1QDN5MjM5ADMwAjMwUzLwQzLxYzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/c/bce/wZwpmL1QjNxcTM2YzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL2czLygzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
①證令,則,即2 =1故有成立。
![冪集公理](/img/9/053/wZwpmLxQDMykzM4gDN3UzM1UTM1QDN5MjM5ADMwAjMwUzL4QzL3UzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![冪集公理](/img/3/a25/wZwpmLxQzN3IDN2YzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL2czLyYzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/c/7f0/wZwpmLwgzM0AzN4ADN3UzM1UTM1QDN5MjM5ADMwAjMwUzLwQzL1MzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/1/129/wZwpmL3MDNwQTMzQzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0MzLyQzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![冪集公理](/img/6/8af/wZwpmLxIjMyITMyUTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL1kzL0UzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/5/dcb/wZwpmLwMTN5EzM4YTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2kzLzAzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/8/023/wZwpmL0UzN1UDO2gTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzLyAzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![冪集公理](/img/8/023/wZwpmL0UzN1UDO2gTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzLyAzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![冪集公理](/img/6/ab9/wZwpmL2IDMwkzM2AzN2UzM1UTM1QDN5MjM5ADMwAjMwUzLwczLxMzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
②證設,且,即若A有k個元,則已知有2 個元。今證。設B有k+1個元的集合,C∈B且則A有k個元,故有個子集,由定理1知,有子集個數是的子集個數的二倍,即有個子集 。
下面給出冪集的一般性質的定理。
定理3
![冪集公理](/img/e/3a1/wZwpmL0AjM4ETN0ITO2UzM1UTM1QDN5MjM5ADMwAjMwUzLykzL2IzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
(1)
![冪集公理](/img/7/952/wZwpmL4gDO5UDN2UTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL1kzL4IzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
(2)
![冪集公理](/img/0/f27/wZwpmL3AzMycjNyYDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2gzL0MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(3)
![冪集公理](/img/5/d9c/wZwpmLwAzM0YTO3EjN2UzM1UTM1QDN5MjM5ADMwAjMwUzLxYzL0YzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
(4)
![冪集公理](/img/6/b84/wZwpmL4gDNyUDNwczM3UzM1UTM1QDN5MjM5ADMwAjMwUzL3MzL3UzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(5)
![冪集公理](/img/0/2ae/wZwpmLzEDNyMDM1MDM3UzM1UTM1QDN5MjM5ADMwAjMwUzLzAzL1UzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(6)
![冪集公理](/img/9/004/wZwpmL2QzM4EjM3QzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL1AzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
(7)
![冪集公理](/img/3/3b8/wZwpmL4AzM5YzM1MDN3UzM1UTM1QDN5MjM5ADMwAjMwUzLzQzL2czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
(8)
![冪集公理](/img/c/bbd/wZwpmLxcTOyQjN4cTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL3UzLyAzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(9)
![冪集公理](/img/0/209/wZwpmLyUTN4UzNzQDM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0AzLyUzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(10)
![冪集公理](/img/e/e19/wZwpmLygDM3EDMwQTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0kzL1MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
(11)
下面討論冪集與傳遞集的密切關係,首先給出如下定義。
![冪集公理](/img/f/2d8/wZwpmLwcTOyADO5cTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL3UzLwczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![冪集公理](/img/c/bd4/wZwpmL1czNwcTN1YDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2gzL1gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/b/cfe/wZwpmL3IjM1ETO2QTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0kzLwYzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
定義3對於集合A中任何集合x和y,若 且 ,則 ,稱A為傳遞集。
![冪集公理](/img/5/1b5/wZwpmLzIDO2UzMxUDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL1gzL1EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
由定義可知, ,可見,要說明A是個傳遞集,只要用下面三種論述之一成立時,便可斷定A是傳遞集:
![冪集公理](/img/f/ca5/wZwpmLxATO5kDO1QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzczLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![冪集公理](/img/a/3f8/wZwpmL0IzNzYTN3ADO2UzM1UTM1QDN5MjM5ADMwAjMwUzLwgzL4IzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![冪集公理](/img/7/b3d/wZwpmL1EjN1UDO0QzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL1AzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
① ,② ,③
![冪集公理](/img/5/1b5/wZwpmLzIDO2UzMxUDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL1gzL1EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
因為它們都等價於 這個性質。
![冪集公理](/img/4/15c/wZwpmL3YTN5QDN0UzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1czLxUzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
定理4對於傳遞集A,有 。
![冪集公理](/img/f/a57/wZwpmL1ETM1YzNxQDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzL1MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
證明: 因為
![冪集公理](/img/e/947/wZwpmLycDMzMjM4MzM3UzM1UTM1QDN5MjM5ADMwAjMwUzLzMzLxEzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
定理5集合A是傳遞集若且唯若 。
![冪集公理](/img/0/d1a/wZwpmL2gTM0EzNygTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2MzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
定理6集合A是一個傳遞集若且唯若 是傳遞集。
定理7每個自然數是個傳遞集。
![冪集公理](/img/b/933/wZwpmLyIjNxcTM4kTNwMDN0UTMyITNykTO0EDMwAjMwUzL5UzL0QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
定理8集合 是個傳遞集 。