The Stacks project

Lemma 12.8.1. Let $\mathcal{C}$ be a preadditive category. Let $S$ be a left or right multiplicative system. There exists a unique preadditive structure on $S^{-1}\mathcal{C}$ such that the localization functor $Q : \mathcal{C} \to S^{-1}\mathcal{C}$ is additive.

Proof. We will prove this in the case $S$ is a left multiplicative system. The case where $S$ is a right multiplicative system is dual. Suppose that $X, Y$ are objects of $\mathcal{C}$ and that $\alpha , \beta : X \to Y$ are morphisms in $S^{-1}\mathcal{C}$. According to Categories, Lemma 4.27.5 we may represent these by pairs $s^{-1}f, s^{-1}g$ with common denominator $s$. Suppose that we have a pre-additive structure on $S^{-1}\mathcal{C}$ and that $Q$ is additive. Then

\begin{align*} s^{-1}f+s^{-1}g & =Q(s)^{-1}\circ Q(f)+Q(s)^{-1}\circ Q(g)\\ & =Q(s)^{-1}\circ (Q(f)+Q(g))\\ & =Q(s)^{-1}\circ Q(f+g)\\ & =s^{-1}(f+g). \end{align*}

Hence, the pre-additive structure on $S^{-1}\mathcal{C}$ is unique if it exists. Correspondingly, we define $\alpha + \beta $ to be the equivalence class of $s^{-1}(f + g)$ and in the rest of the proof we show that this is well defined and that composition is bilinear. Once this is done it is clear that $Q$ is an additive functor.

Let us show construction above is well defined. An abstract way of saying this is that filtered colimits of abelian groups agree with filtered colimits of sets and to use Categories, Equation (4.27.7.1). We can work this out in a bit more detail as follows. Say $s : Y \to Y_1$ and $f, g : X \to Y_1$. Suppose we have a second representation of $\alpha , \beta $ as $(s')^{-1}f', (s')^{-1}g'$ with $s' : Y \to Y_2$ and $f', g' : X \to Y_2$. By Categories, Remark 4.27.7 we can find a morphism $s_3 : Y \to Y_3$ and morphisms $a_1 : Y_1 \to Y_3$, $a_2 : Y_2 \to Y_3$ such that $a_1 \circ s = s_3 = a_2 \circ s'$ and also $a_1 \circ f = a_2 \circ f'$ and $a_1 \circ g = a_2 \circ g'$. Hence we see that $s^{-1}(f + g)$ is equivalent to

\begin{align*} s_3^{-1}(a_1 \circ (f + g)) & = s_3^{-1}(a_1 \circ f + a_1 \circ g) \\ & = s_3^{-1}(a_2 \circ f' + a_2 \circ g') \\ & = s_3^{-1}(a_2 \circ (f' + g')) \end{align*}

which is equivalent to $(s')^{-1}(f' + g')$.

Fix $s : Y \to Y'$ and $f, g : X \to Y'$ with $\alpha = s^{-1}f$ and $\beta = s^{-1}g$ as morphisms $X \to Y$ in $S^{-1}\mathcal{C}$. To show that composition is bilinear first consider the case of a morphism $\gamma : Y \to Z$ in $S^{-1}\mathcal{C}$. Say $\gamma = t^{-1}h$ for some $h : Y \to Z'$ and $t : Z \to Z'$ in $S$. Using LMS2 we choose morphisms $a : Y' \to Z''$ and $t' : Z' \to Z''$ in $S$ such that $a \circ s = t' \circ h$. Picture

\[ \xymatrix{ & & Z \ar[d]^ t \\ & Y \ar[r]^ h \ar[d]^ s & Z' \ar[d]^{t'} \\ X \ar[r]^{f, g} & Y' \ar[r]^ a & Z'' } \]

Then $\gamma \circ \alpha = (t' \circ t)^{-1}(a \circ f)$ and $\gamma \circ \beta = (t' \circ t)^{-1}(a \circ g)$. Hence we see that $\gamma \circ (\alpha + \beta )$ is represented by $(t' \circ t)^{-1}(a \circ (f + g)) = (t' \circ t)^{-1}(a \circ f + a \circ g)$ which represents $\gamma \circ \alpha + \gamma \circ \beta $.

Finally, assume that $\delta : W \to X$ is another morphism of $S^{-1}\mathcal{C}$. Say $\delta = r^{-1}i$ for some $i : W \to X'$ and $r : X \to X'$ in $S$. We claim that we can find a morphism $s' : Y' \to Y''$ in $S$ and morphisms $a'', b'' : X' \to Y''$ such that the following diagram commutes

\[ \xymatrix{ & & & Y \ar[d]^ s \\ & X \ar[rr]^{f, g, f + g} \ar[d]^ r & & Y' \ar[d]^{s'} \\ W \ar[r]^ i & X' \ar[rr]^{a'', b'', a'' + b''} & & Y'' } \]

Namely, using LMS2 we can first choose $s_1 : Y' \to Y_1$, $s_2 : Y' \to Y_2$ in $S$ and $a : X' \to Y_1$, $b : X' \to Y_2$ such that $a \circ r = s_1 \circ f$ and $b \circ r = s_2 \circ f$. Then using that the category $Y'/S$ is filtered (see Categories, Remark 4.27.7), we can find a $s' : Y' \to Y''$ and morphisms $a' : Y_1 \to Y''$, $b' : Y_2 \to Y''$ such that $s' = a' \circ s_1$ and $s' = b' \circ s_2$. Setting $a'' = a' \circ a$ and $b'' = b' \circ b$ works. At this point we see that the compositions $\alpha \circ \delta $ and $\beta \circ \delta $ are represented by $(s' \circ s)^{-1}(a'' \circ i)$ and $(s' \circ s)^{-1}(b'' \circ i)$. Hence $\alpha \circ \delta + \beta \circ \delta $ is represented by $(s' \circ s)^{-1}(a'' \circ i + b'' \circ i) = (s' \circ s)^{-1}((a'' + b'') \circ i)$ which by the diagram again is a representative of $(\alpha + \beta ) \circ \delta $. $\square$


Comments (3)

Comment #467 by Nuno on

In the equation , should be . Also, in the first diagram should be .

Comment #8553 by on

In the statement, one could change "canonical" by "unique." Here's the proof of the uniqueness:

Suppose is an additive functor of preadditive categories. Let be objects of and be morphisms in . According to Categories, Lemma 4.27.5, we may represent these by pairs with common denominator . Then Hence, the preadditive structure in is determined by that of .

The reason I find interesting to add the uniqueness to this lemma is to guarantee the uniqueness in the statement of Derived Categories, Lemma 13.5.6.

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