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Let $f:R \rightarrow R'$ be a ring homomorphism that is epic in the category of rings. Let $M,N$ be $R'$-modules. Why is it that a homorphism $h:M \rightarrow N$ is $R'$-linear if and only if it is $R$-linear? I have used this result in specific cases in the past (and know why it's true),e.g., if $f$ is the usual map of $R$ to $S^{-1}R$,$S$ a multiplicatively closed subset of $R$, but never in the generality I stated. But now I need the general result and would like to know why it works. A reference or a sketch for a proof would be appreciated.

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Recall the characterization of epimorphisms in the category of rings (see for example this question on MathOverflow):

Silver-Mazet-Isbell Zigzag Lemma for rings. Let $f\colon R\to S$ be a ring homomorphism. Then $f$ is an epimorphism in the category of rings if and only if for every $s\in S$ there exist matrices $C$, $D$, and $E$, where $C$ is $1\times n$, $D$ is $n\times n$, $E$ is $n\times 1$; $C$ and $E$ have coefficients in $S$; $CD$, $D$, and $DE$ have coefficients in $f(R)$; and $s=CDE$. This is called a "zigzag in $S$ over $f(R)$ with value $s$."

For simplicity, suppose that $R\subseteq R'$ and that $f$ is the canonical inclusion. Now I'm going to be a bit sloppy below, by identifying $M$ with its image in obvious matrix rings, but I think the idea carries through:

Suppose $h\colon M\to N$ is $R$-linear, and let $m\in M$, $b\in R'$. We want to show that $h(bm) = bh(m)$. Since $R\hookrightarrow R'$ is an epimorphism, there is a zigzag in $R'$ over $R$ with value $b$, $b=CDE$, where $D$, $CD$, and $CE$ are matrices with coefficients in $R$. Since $h$ is $R$ linear, we have: \begin{align*} h(bm) &= h(CDEm)\\ &= CDh(Em) &\qquad&\mbox{(since $CD$ has coefficients in $R$)}\\ &= Ch(DEm) &&\mbox{(since $D$ has coefficients in $R$)}\\ &= C(DEh(m)) &&\mbox{(since $DE$ has coefficients in $R$)}\\ &= (CDE)h(m)\\ &= bh(m). \end{align*} As I said, that's a bit sloppy, since in the middle I'm actually dealing with first with $\mathcal{M}_{1\times n}(M)$ as an $\mathcal{M}_{1\times n}(R')$ module, with the induced map $h$; and later I'm dealing with $\mathcal{M}_{n\times n}(M)$ and $\mathcal{M}_{n\times 1}(M)$ over suitable matrix rings. But essentially that is what is going on.

(You can also probably see why it's called a "zigzag": you zigzag between $CD$ and $DE$ to pull the entire scalar out.)

Isbell's Zigzag Lemma (characterizing elements that are in the dominion of a subalgebra in a large class of subalgebras) can be found in

  • Isbell, John R. Epimorphisms and dominions. 1966 Proc. Conf. Categorical Algebra (La Jolla, Calif., 1965) pp. 232-246 Springer, New York, MR0209202 (35 #105a)

The statement of the Zigzag Lemma for rings is incorrect in that paper. A correct version appears in:

More generally, the collection of all $s\in S$ for which there is a Zigzag in $S$ over $f(R)$ with value $s$ is called the dominion of $f(R)$ in $S$. It is precisely the subring of $S$ given by: $$\mathrm{dom}_S(f(R)) = \Bigl\{ s\in S\Bigm| \forall T,\ \forall g,h\colon S\to T( g|_{f(R)} = h|_{f(R)}\Rightarrow g(s)=h(s))\Bigr\}.$$ The argument above shows that any module map that is $R$ linear will necessarily be $\mathrm{dom}_{R'}(f(R))$-linear; in the special case when $\mathrm{dom}_{R'}(f(R)) = R'$, which is precisely the case where $f$ is an epimorphism, the conclusion you want follows.

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Thanks, Arturo. –  Chris Leary Mar 14 '11 at 17:22
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