# On centers of infinite $p$-groups and nilpotent groups

If $G$ is a finite $p$-group, then its center is non-trivial, which forces that $G$ must be nilpotent.

Consider infinite $p$-groups, i.e. infinite groups in which order of every element is some power of (a common prime) $p$. In such a group, we can not say that center is non-trivial (see this example). In particular, I could say that upper central series of $G$ do not terminates at $G$.

But, this troubles me to conclude whether $G$ is nilpotent or not? I would like to clarify following things related to this. Please help me.

Question 1. What is (are) the definition(s) of infinite nilpotent group which can be looked as generalizations of finite nilpotent groups?

Question 2. If $G$ is an infinite $p$-group, then what conditions on $G$ would guarantee that $Z(G)$ is non-trivial?

• The usual definition for infinite groups is the same as for finite groups, in terms of the upper central series. – Qiaochu Yuan Oct 22 '15 at 6:22
• I am now aware of any sensible answer to Question 2. What type of conditions did you have in mind? – Derek Holt Oct 22 '15 at 8:16
• I couldn't thought about it because of the Tarski-Monster groups. – Groups Oct 22 '15 at 12:13