# Upper central series members are quotient-powering-invariant

## Statement

Suppose $G$ is a group. Denote by $Z^{(0)}(G),Z^{(1)}(G),Z^{(2)}(G),\dots$ the members of the upper central series of $G$, all of which are subgroups of $G$. Here, $Z^{(0)}(G)$ is the trivial subgroup, $Z^{(1)}(G)$ is the center, $Z^{(2)}(G)$ is the second center, and so on. Then, all the subgroups $Z^{(i)}(G)$, for nonnegative integers $i$, are quotient-powering-invariant subgroups of $G$. In particular, since quotient-powering-invariant implies powering-invariant, they are all powering-invariant subgroups of $G$.

The result can in fact be extended to the transfinite upper central series.

## Proof

### Proof for the finite part

The proof follows directly by combining Facts (1) and (2), and using the principle of mathematical induction (just a single application would get us to the second center).

### Proof for the transfinite upper central series

This basically follows the same way, but we need to use transfinite induction instead. The additional ingredient we need is the argument for limit ordinals, and this basically follows from Fact (3).