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其实只是 Terence Tao 大神的定义写得稍微有一点绕,整体还是好理解的。

Wikipedia: Sequence

Formally, a sequence can be defined as a function whose domain is either the set of the natural numbers (for infinite sequences) or the set of the first n natural numbers (for a sequence of finite length n)

也就是说,sequence (ai)iI 和 function f:I{aiiI} 是等价的。f:iai 也就是 (ai)iI.

Axiom 3.10 (Power set axiom). Let X and Y be sets. Then there exists a set, denoted YX, which consists of all the functions from X to Y, thus

fYXf is a function with domain X and range Y
  • 亦即 YX={ff is a function with domain X and range Y}
  • 注意是 “以 Y 为 range” 而不是 “以 Y 为 codomain”

你结合上面 sequence 的结论就可以看出,YX 其实也可看成是一个 sequence 的集合。大神 infinite Cartesian products 的定义就这么写的:

Definition 8.4.1 (Infinite Cartesian products). Let I be an index set (possibly infinite), and for each iI let Xi be a set. We then define the Cartesian product iIXi to be the set

iIXi={(xi)iI(jIXj)IiI,xiXi}

这个定义是兼容 “有限笛卡尔积” 的。举个例子:X1={a3},X2={a2},X3={a1}。明显 X1×X2×X3={(a3,a2,a1)} 就这么一种组合。按照上面的定义:

  • I={1,2,3}
  • jIXj={a1,a2,a3}
  • (jIXj)I 是所有 f:{1,2,3}{a1,a2,a3} 的集合
  • 条件 iI,xiXi 其实应该理解为 iI,f(i)Xi。这样的 f 只有一个,即 f(1)=x1=a3,f(2)=x2=a3,f(3)=x3=a1
  • 这个 f 亦即 sequence (a3,a2,a1)

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