Essentials of Programming Languages 3th 的一道练习题

Question

Definition 1.1.2 Define the set S to be the smallest set contained in N and satisfying the following two properties:

1. 0 ∈ S, and
2. if n ∈ S, then n + 3 ∈ S

这里说定义好了0,3,6...这样的集合，然后说smallest set是必须的否则有很多这样的集合?
练习1.3

Find a set T of natural numbers such that 0 ∈ T, and whenever n ∈ T,
then n + 3 ∈ T, but T != S, where S is the set defined in definition 1.1.2.

举个T的例子?

Answer 1

所有自然数集合N就可以。