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I want to understand the structure of monomorphisms/epimorphisms in the category of local rings (with local homomorphisms), or dually in the category of local schemes. Let $LR$ denote this category.

• Every monomorphism is injective.

Proof: Let $R \to S$ be a monomorphism and $a,b \in R$ mapped to the same element. Regard $a,b$ as ring maps $R[t] \to R$. Let $\mathfrak{p},\mathfrak{q}$ the preimages of the maximal ideal $\mathfrak{m}$. Then we get two morphisms of local rings $R[t]_{\mathfrak{p}} \times_S R[t]_{\mathfrak{q}} \to R$ which agree when composed with $R \to S$, thus they are equal. Evaluating at $(t,t)$ gives $a=b$. There is a shorter proof which uses $R[t]_{(t)+\mathfrak{m}}$.

1. Is every monomorpism regular? (i.e. the equalizer of some morphism pair)

2. Is every monomorphism extremal? (i.e. $i=ep$ with $p$ epi implies $p$ iso)

3. Is every epimorphism, which is also injective, and isomorphism?

4. Is every epimorphism surjective?

Clearly we have 4 => 3 <=> 2 and 1 => 2. Since in $LR$ every morphism can be factorized as $ep$ where $p$ is surjective and $e$ is injective, we also have 3 => 4.

In the category of rings, 1,2,3,4 are false (see this discussion). However, the counterexamples don't carry over to $LR$. In the "category theory bible" (joy of cats!) you can find nothing about local rings. The questions are partially motivated by Anton's problem about coequalizers in the category of (local) schemes.

In Lazards article about flat epimorphisms (pdf link), there is an example (1.6) in which 4 fails. I have not understood it yet. What is $Z$ there? Perhaps $B$ is a localization of $k[T,Z]$ and not $k[T]$? But I don't believe that $f$ is well-defined.

If the answers turn out to be "no":

• Is there a nice description for the regular monomorphisms? What about field extensions?

If $L/K$ is a finite galois extension, then $K \to L$ is a regular monomorphism iff $L/K$ is cyclic (see Brians comments).

PS: This is a dublicate of my old question, but the old one will disappear (see the comments there).

Question Merged by Anton Geraschenko♦♦
1

# monomorphisms and epimorphisms of local rings

I want to understand the structure of monomorphisms/epimorphisms in the category of local rings (with local homomorphisms), or dually in the category of local schemes. Let $LR$ denote this category.

• Every monomorphism is injective.

Proof: Let $R \to S$ be a monomorphism and $a,b \in R$ mapped to the same element. Regard $a,b$ as ring maps $R[t] \to R$. Let $\mathfrak{p},\mathfrak{q}$ the preimages of the maximal ideal $\mathfrak{m}$. Then we get two morphisms of local rings $R[t]_{\mathfrak{p}} \times_S R[t]_{\mathfrak{q}} \to R$ which agree when composed with $R \to S$, thus they are equal. Evaluating at $(t,t)$ gives $a=b$. There is a shorter proof which uses $R[t]_{(t)+\mathfrak{m}}$.

1. Is every monomorpism regular? (i.e. the equalizer of some morphism pair)

2. Is every monomorphism extremal? (i.e. $i=ep$ with $p$ epi implies $p$ iso)

3. Is every epimorphism, which is also injective, and isomorphism?

4. Is every epimorphism surjective?

Clearly we have 4 => 3 <=> 2 and 1 => 2. Since in $LR$ every morphism can be factorized as $ep$ where $p$ is surjective and $e$ is injective, we also have 3 => 4.

In the category of rings, 1,2,3,4 are false (see this discussion). However, the counterexamples don't carry over to $LR$. In the "category theory bible" (joy of cats!) you can find nothing about local rings. The questions are partially motivated by Anton's problem about coequalizers in the category of (local) schemes.

In Lazards article about flat epimorphisms (pdf link), there is an example (1.6) in which 4 fails. I have not understood it yet. What is $Z$ there? Perhaps $B$ is a localization of $k[T,Z]$ and not $k[T]$? But I don't believe that $f$ is well-defined.

If the answers turn out to be "no":

• Is there a nice description for the regular monomorphisms? What about field extensions?

If $L/K$ is a finite galois extension, then $K \to L$ is a regular monomorphism iff $L/K$ is cyclic (see Brians comments).

PS: This is a dublicate of my old question, but the old one will disappear (see the comments there).