# Inverse limits of groups of units of a ring.

Can someone enlighten me on the following isomorphism described in "Exercises in Modern Algebra" by Professor A. Hattori (in Japanese)?

Let $\lbrace R^{\mu} \rbrace$ be an inverse system of rings (with units.) Then $(\varprojlim (R^{\mu}))^{\times} \cong \varprojlim (R^{\mu})^{\times}$ holds where $^{\times}$ denotes the group of units. The verification of this fact is easy so the proof is omitted.

I was not able to come up with a proof, so I googled and found one by Professor Igusa (Corollary 2.7. of http://people.brandeis.edu/~igusa/Math101aF07/Math101a_notesBall.pdf).

I know I have to be satisfied with this, but I don't think the proof is "easy": the techniques used hardly fit into first-year algebra courses.

Is there an easy trick that Professor Hattori had in mind which I am overlooking?

-
I think the isomorphism is pretty clear if we view the inverse limit as the subring of the direct product satisfying the appropriate transition condition on the coordinates. –  anon Nov 25 '12 at 6:49
@anon: Maybe I am dumb: I don't recognise it as pretty clear. Can you be more specific on this? –  eltonjohn Nov 25 '12 at 6:59
You can use the > character to create a block quote, like in email. P.S. Would the ring-theory tag be appropriate here? –  Rahul Narain Nov 25 '12 at 9:24
Hello eltonjohn. Have you considered registering your account? I believe this is also you. –  Matt N. Nov 25 '12 at 9:44
@eltonjohn "it's me" is correct. Well, perhaps the moderators can help, I notified them. It might be more convenient for you to have your questions all associated to one user. (if you want to look at them again in the future) –  Matt N. Nov 25 '12 at 11:08

It is standard in the proof of existence of inverse limits to provide a concrete construction of the inverse limit as a subring of the direct product of the system's rings satisfying the appropriate transition conditions. That is, if $\varphi_{\mu\lambda}:R^\mu\to R^\lambda$ are the transition morphisms of the system,

$$\varprojlim R^\mu\cong\{(r^\mu):\underbrace{\forall\mu,\lambda,~\varphi_{\mu\lambda}(r^\mu)=r^\lambda}_{\text{transition conditions}}\}\subseteq \prod_\mu R^\mu. \tag{\square}$$

We use $\subseteq$ to denote subring above. It is straightforward to check that the middle set is a subring; for convenience we will refer to it by $L$ (for "limit").

For good measure, you may want to convince yourself that (a) the assignment of groups of units to rings is functorial in the latter, and (b) this functor distributes through arbitrary direct products.

If $(r^\mu)\in L$ is a unit, then there is a $(s^\mu)\in L$ for which $1_{L}=(1_{R^\mu})=(r^\mu)(s^\mu)=(r^\mu s^\mu)$ and hence the coordinate $r^\mu$ is invertible for each $\mu$. Thus if $(r^\mu)\in L^\times$ then $(r^\mu)\in\prod_\mu (R^\mu)^\times$; but since $(r^\mu)$ is in $L$ it satisfies the transition conditions, and hence it is contained in the subgroup of $\prod_\mu(R^\mu)^\times$ which is isomorphic to $\varprojlim(R^\mu)^\times$ as per $(\square)$ (although working in a different category, the idea works just the same). Conversely, if $(r^\mu)$ is in the designated subgroup of $\prod_\mu(R^\mu)^\times$ (itself a subset of $\prod_\mu R^\mu$) which is isomorphic to $\varprojlim (R^\mu)^\times$, then each coordinate $r^\mu$ is a unit in $R^\mu$ with inverse say $s^\mu$, and so $(r^\mu)(s^\mu)=1_L$ shows $(r^\mu)$ is invertible and in $L$ (again since it satisfies the TCs).

Thus the copy of $\varprojlim (R^\mu)^\times$ sitting inside $\prod_\mu(R^\mu)^\times$ sitting inside $\prod_\mu R^\mu$ is equal to the units of the copy of $\varprojlim R^\mu$ sitting inside $\prod_\mu R^\mu$.

-
Thank you very much indeed. Yes, I was trying to come up with a proof that deploys direct construction of limits. Your explanation is wonderful: I didn't have to read between the lines! –  eltonjohn Nov 25 '12 at 9:30
Whether this counts as an "easy trick" is up to you, but the group of units is a functor $\text{Ring} \to \text{Grp}$ which has a left adjoint, namely the group ring construction. Any functor with a left adjoint preserves limits.