5. Sets集合
5.1Introduction介绍 Set集合 n. [set]
Definition: A group of objects represented as a unit. The objects are called elements.集合代表一个单元的一组对象。每个对象称为元素。
Notation: (1) Explicit: {element 1,element 2,…,element n},this is a set of n elements.集合用列举法表示为: {元素1,元素2,…,元素n},该集合是一个含有n个元素的集合。
For example,{1,3,5,7,9} is a set with 5 elements.
例如,{1,3,5,7,9}集合有5个元素。
(2) Implicit: {x | …(condition)…},read as “the set of all x such that (the condition is true)”. For example {x | x is odd and less than 10} is read as “the set of all x such that x is odd and less than 10”. To be represented explicitly,this is {1,3,5,7,9}.集合用描述法表示为: {x|条件},读作“包含所有满足条件的元素x的集合”。例如{x|x是小于10的奇数},表示的是“包含所有小于10的奇数的集合”。用列举法表示,为{1,3,5,7,9}。
Properties: 1. Duplication does not affect the uniqueness of a set.重复的元素不改变集合的独特性。
For example,{1,2,2,3,3,3}={1,2,3}.
2. Ordering does not affect the uniqueness of a set.排序不改变集合的独特性。
For example,{1,2,3}={3,2,1}.
3. A set can have both finite and infinite number of elements. In the examples below,A has a finite number of elements,and B has an infinite number of elements.集合的元素个数可以为有限的,亦可以为无限的。在下面的例子中,集合A的元素个数为有限的,集合B的元素个数为无限的。
A={0}={x | x is neither positive nor negative} (There are many other ways to define it implicitly,but the explicit definition,{0},will always be the same.)本例用列举法比用描述法更方便。
B={ x | x>0} (There are infinitely many positive numbers.)正数的个数是无限的。
Element元素
n. [5elmnt]
Definition: Refers to a distinct member that makes up the set.元素是指集合中与众不同的每个对象。
Notation: If a is an element of set A,then we write a∈A.
Properties: Elements do not have to be numbers.元素不一定是数字。
Examples: Consider the following sets:
A={1,2,3,3,4,5}
B={1,2,3,a,b,c,apple,pear}
C={x | x is even}
1. The elements of A are 1,2,3,4,and 5. Set A equals to {1,2,3,4,5}. See Property 1 of set.
2. The elements of B are 1,2,3,a,b,c,apple,and pear.
3. Some elements of C are 0,2,4,6,8,….
Cardinality基数
n. [5kɑ.dn5l..ti]
Definition: Refers to the size,or the number of distinct elements,of a set.基数指的是集合的大小,即元素的个数。
Notation: The cardinality of set A is denoted as |A|.
Properties: 1. For sets that have infinitely many elements,the cardinality is infinity.对于有无穷多元素的集合,基数为无穷。
2. For the empty set,the cardinality is 0. 空集的基数为0。
Examples: Consider the following sets:
A={1,2,3,4,5}
B={1,2,2,3,3,3,4,4,4,4,5,5,5,5,5}
C={x | x is even}
1. |A|=5.
2. |B|=5.
3. |C|=∞.
Empty Set空集
n. [emp.ti5set]
Definition: The only set that has 0 elements. i.e. The only set whose cardinality is 0.空集是**的只有0个元素的集合,是**的基数为0的集合。
Notation: or {}.
Properties: ||=0,and ≠{0}.