[seqfan] Re: Possible number of units in a ring

[Max Alekseyev]

On Tue, Apr 17, 2012 at 12:16 PM, Charles Greathouse
<charles.greathouse at case.edu> wrote:
> Sequence A070932 is described in a comment:
>
> By considering the ring Z_n and the finite fields GF(q) this sequence
> contains the values of Euler function (A000010) and prime powers - 1 .
> By taking direct products if n and m belong to the sequence so is m*n.
>
> Do these conditions suffice to describe all terms of the sequence?

Probably not.
The first comment just gives examples of most "simple" rings. But
there exist more complicated rings.
Cf. A037289

Regards,
Max