OK, so for this guy, domainofdefinition, which is plane minus the origin with the origin removed is not simply connected.
好滴，目前这里，在有定义的区域里，减掉原点，即移去原点，就不是单连通的。
2
People are likely to overlook in teaching the essential factor that the domainofdefinition of function concept is just the nonempty set of number.
在函数概念教学中 ，往往容易忽略其定义域为非空数集这一要素 。
3
Some common mistakes made on function problem are as follows, the concept unclear, the neglect domainofdefinition, the contempt value territory and the omission concealment condition.