If p is a set of integers and 3 is in p, is every positive multiple of 3 in p?
1。if x is in p,then x+3 is in p.
2.  if x is in p, then x-3 is in p.

不明白答案怎么是A

最特殊的一个数恰就是楼上举的。

如果x是1或者2，那又怎么证明3的每个正倍数都在p中呢？

我觉得应该是e

有书的朋友可以看钱永强数学难题精解的P130页。