What I didn’t understand in Chapter One

[rasin-tsukuba]

I’m not sure if I understand that there is a problem or there is an error in the book. I would like to ask everyone, please feel free to enlighten me.

1. In Example 1.1.1, maybe \frac{2}{3} \notin \mathbb{Z}?
   
2. In Example 1.1.1, I think A-B is a set but not an element, but \in should be used in a relationship between elements and sets. Maybe it should be A-B \in {1, 2}?
   
3. In Lemma 1.1.3, is this a typo error?
   

Hope to get the answer, thank you very much!