# the localization of R at P is a regular local ring then R is regular local ring

We know the fact that if $R$ is a regular local ring then $R_{P}$ the localization of $R$ at $P$, $P\in\mathrm{Spec}(R)$ is a regular local ring.

So, I wonder the converse is true or not? My counter-example is taking $R=\mathbb Z$.

-If $P=(0)$ then $R_{P}$ is a field so $R_{P}$ is a regular local ring

-If $P=(p)$, $p$ is a prime number then $\dim R_{p}=1=v(PR_{P})$ (generate by $\frac{p}{1}$) so it is also a regular local ring.

However, $\mathbb Z$ is not a regular local ring. Can you check that to me please?

• I know it is not true. I just wanna know my counter example is true or false. – Soulostar Jan 9 '17 at 9:58
• Because I do not know if the localization of R at P is a regular local ring with every P in Spec(R) then is R a regular local ring? It is not true and I try to find the example of that. – Soulostar Jan 9 '17 at 10:02
• Oh I see your meaning. You mean that $R$ is regular iff $R_{P}$ is regular local ring with every prime ideal $P \in Spec(R)$. But it is just quite similar to my question. The theorem (Serre 1955) is: Suppose that $(R;m)$ is a regular local ring. Then $R_{P}$ is again a regular local ring for every a prime ideal $P \in$R$. I wonder the converse of this theorem is true or not? so we just find some rings which is not local then the converse is not true. – Soulostar Jan 9 '17 at 10:26 • No, it is if every$P\in Spec(R)$the localization of$R$at P is regular local ring then is R regular local ring? – Soulostar Jan 9 '17 at 10:29 • But how about your meaning? Can you give me the example? @user26857 – Soulostar Jan 9 '17 at 10:45 ## 1 Answer Let$I$be a prime ideal of$R_P$, then$I^c$is a prime ideal of$\mathbb{Z}$, so$I^c=q\mathbb{Z}$for some prime number$q$. We have$I=I^{ce}=(q\mathbb{Z})^e=(\frac{q}{1})$. However,$q\mathbb{Z}$is maximal so$I$is also maximal. So an arbitrary nonzero prime ideal of$R_P$is maximal, which means$\dim R_P=1=v(PR_P)\$

• Yes, I see that is what I mention. :D – Soulostar Jan 9 '17 at 8:28
• O...k. So what do need to check? – chí trung châu Jan 9 '17 at 8:28
• I just wanna know it is true or false :). – Soulostar Jan 9 '17 at 8:33
• Well, it's true then – chí trung châu Jan 9 '17 at 8:37