Section 4.22 (05PT): Essentially constant systems

4.22 Essentially constant systems

Let $M : \mathcal{I} \to \mathcal{C}$ be a diagram in a category $\mathcal{C}$. Assume the index category $\mathcal{I}$ is filtered. In this case there are three successively stronger notions which pick out an object $X$ of $\mathcal{C}$. The first is just

\[ X = \mathop{\mathrm{colim}}\nolimits _{i \in \mathcal{I}} M_ i. \]

Then $X$ comes equipped with the coprojections $M_ i \to X$. A stronger condition would be to require that $X$ is the colimit and that there exist an $i \in \mathcal{I}$ and a morphism $X \to M_ i$ such that the composition $X \to M_ i \to X$ is $\text{id}_ X$. An even stronger condition is the following.

Definition 4.22.1. Let $M : \mathcal{I} \to \mathcal{C}$ be a diagram in a category $\mathcal{C}$.

Assume the index category $\mathcal{I}$ is filtered and let $(X, \{ M_ i \to X\} _ i)$ be a cocone for $M$, see Remark 4.14.5. We say $M$ is essentially constant with value $X$ if there exist an $i \in \mathcal{I}$ and a morphism $X \to M_ i$ such that
1. $X \to M_ i \to X$ is $\text{id}_ X$, and
2. for all $j$ there exist $k$ and morphisms $i \to k$ and $j \to k$ such that the morphism $M_ j \to M_ k$ equals the composition $M_ j \to X \to M_ i \to M_ k$.
Assume the index category $\mathcal{I}$ is cofiltered and let $(X, \{ X \to M_ i\} _ i)$ be a cone for $M$, see Remark 4.14.5. We say $M$ is essentially constant with value $X$ if there exist an $i \in \mathcal{I}$ and a morphism $M_ i \to X$ such that
1. $X \to M_ i \to X$ is $\text{id}_ X$, and
2. for all $j$ there exist $k$ and morphisms $k \to i$ and $k \to j$ such that the morphism $M_ k \to M_ j$ equals the composition $M_ k \to M_ i \to X \to M_ j$.

Please keep in mind Lemma 4.22.3 when using this definition.

Which of the two versions is meant will be clear from context. If there is any confusion we will distinguish between these by saying that the first version means $M$ is essentially constant as an ind-object, and in the second case we will say it is essentially constant as a pro-object. This terminology is further explained in Remarks 4.22.4 and 4.22.5. In fact we will often use the terminology “essentially constant system” which formally speaking is only defined for systems over directed sets.

Definition 4.22.2. Let $\mathcal{C}$ be a category. A directed system $(M_ i, f_{ii'})$ is an essentially constant system if $M$ viewed as a functor $I \to \mathcal{C}$ defines an essentially constant diagram. A directed inverse system $(M_ i, f_{ii'})$ is an essentially constant inverse system if $M$ viewed as a functor $I^{opp} \to \mathcal{C}$ defines an essentially constant inverse diagram.

If $(M_ i, f_{ii'})$ is an essentially constant system and the morphisms $f_{ii'}$ are monomorphisms, then for all $i \leq i'$ sufficiently large the morphisms $f_{ii'}$ are isomorphisms. On the other hand, consider the system

\[ \mathbf{Z}^2 \to \mathbf{Z}^2 \to \mathbf{Z}^2 \to \ldots \]

with maps given by $(a, b) \mapsto (a + b, 0)$. This system is essentially constant with value $\mathbf{Z}$ but every transition map has a kernel.

Here is an example of a system which is not essentially constant. Let $M = \bigoplus _{n \geq 0} \mathbf{Z}$ and to let $S : M \to M$ be the shift operator $(a_0, a_1, \ldots ) \mapsto (a_1, a_2, \ldots )$. In this case the system $M \to M \to M \to \ldots $ with transition maps $S$ has colimit $0$ and the composition $0 \to M \to 0$ is the identity, but the system is not essentially constant.

The following lemma is a sanity check.

Lemma 4.22.3. Let $M : \mathcal{I} \to \mathcal{C}$ be a diagram. If $\mathcal{I}$ is filtered and $M$ is essentially constant as an ind-object, then $X = \mathop{\mathrm{colim}}\nolimits M_ i$ exists and $M$ is essentially constant with value $X$. If $\mathcal{I}$ is cofiltered and $M$ is essentially constant as a pro-object, then $X = \mathop{\mathrm{lim}}\nolimits M_ i$ exists and $M$ is essentially constant with value $X$.

Proof. Omitted. This is a good excercise in the definitions. $\square$

Remark 4.22.4. Let $\mathcal{C}$ be a category. There exists a big category $\text{Ind-}\mathcal{C}$ of ind-objects of $\mathcal{C}$. Namely, if $F : \mathcal{I} \to \mathcal{C}$ and $G : \mathcal{J} \to \mathcal{C}$ are filtered diagrams in $\mathcal{C}$, then we can define

\[ \mathop{\mathrm{Mor}}\nolimits _{\text{Ind-}\mathcal{C}}(F, G) = \mathop{\mathrm{lim}}\nolimits _ i \mathop{\mathrm{colim}}\nolimits _ j \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(F(i), G(j)). \]

There is a canonical functor $\mathcal{C} \to \text{Ind-}\mathcal{C}$ which maps $X$ to the constant system on $X$. This is a fully faithful embedding. In this language one sees that a diagram $F$ is essentially constant if and only if $F$ is isomorphic to a constant system. If we ever need this material, then we will formulate this into a lemma and prove it here.

Remark 4.22.5. Let $\mathcal{C}$ be a category. There exists a big category $\text{Pro-}\mathcal{C}$ of pro-objects of $\mathcal{C}$. Namely, if $F : \mathcal{I} \to \mathcal{C}$ and $G : \mathcal{J} \to \mathcal{C}$ are cofiltered diagrams in $\mathcal{C}$, then we can define

\[ \mathop{\mathrm{Mor}}\nolimits _{\text{Pro-}\mathcal{C}}(F, G) = \mathop{\mathrm{lim}}\nolimits _ j \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(F(i), G(j)). \]

There is a canonical functor $\mathcal{C} \to \text{Pro-}\mathcal{C}$ which maps $X$ to the constant system on $X$. This is a fully faithful embedding. In this language one sees that a diagram $F$ is essentially constant if and only if $F$ is isomorphic to a constant system. If we ever need this material, then we will formulate this into a lemma and prove it here.

Example 4.22.6. Let $\mathcal{C}$ be a category. Let $(X_ n)$ and $(Y_ n)$ be inverse systems in $\mathcal{C}$ over $\mathbf{N}$ with the usual ordering. Picture:

\[ \ldots \to X_3 \to X_2 \to X_1 \quad \text{and}\quad \ldots \to Y_3 \to Y_2 \to Y_1 \]

Let $a : (X_ n) \to (Y_ n)$ be a morphism of pro-objects of $\mathcal{C}$. What does $a$ amount to? Well, for each $n \in \mathbf{N}$ there should exist an $m(n)$ and a morphism $a_ n : X_{m(n)} \to Y_ n$. These morphisms ought to agree in the following sense: for all $n' \geq n$ there exists an $m(n', n) \geq m(n'), m(n)$ such that the diagram

\[ \xymatrix{ X_{m(n, n')} \ar[rr] \ar[d] & & X_{m(n)} \ar[d]^{a_ n} \\ X_{m(n')} \ar[r]^{a_{n'}} & Y_{n'} \ar[r] & Y_ n } \]

commutes. After replacing $m(n)$ by $\max _{k, l \leq n}\{ m(n, k), m(k, l)\} $ we see that we obtain $\ldots \geq m(3) \geq m(2) \geq m(1)$ and a commutative diagram

\[ \xymatrix{ \ldots \ar[r] & X_{m(3)} \ar[d]^{a_3} \ar[r] & X_{m(2)} \ar[d]^{a_2} \ar[r] & X_{m(1)} \ar[d]^{a_1} \\ \ldots \ar[r] & Y_3 \ar[r] & Y_2 \ar[r] & Y_1 } \]

Given an increasing map $m' : \mathbf{N} \to \mathbf{N}$ with $m' \geq m$ and setting $a'_ i : X_{m'(i)} \to X_{m(i)} \to Y_ i$ the pair $(m', a')$ defines the same morphism of pro-systems. Conversely, given two pairs $(m_1, a_1)$ and $(m_1, a_2)$ as above then these define the same morphism of pro-objects if and only if we can find $m' \geq m_1, m_2$ such that $a'_1 = a'_2$.

Remark 4.22.7. Let $\mathcal{C}$ be a category. Let $F : \mathcal{I} \to \mathcal{C}$ and $G : \mathcal{J} \to \mathcal{C}$ be cofiltered diagrams in $\mathcal{C}$. Consider the functors $A, B : \mathcal{C} \to \textit{Sets}$ defined by

\[ A(X) = \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(F(i), X) \quad \text{and}\quad B(X) = \mathop{\mathrm{colim}}\nolimits _ j \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(G(j), X) \]

We claim that a morphism of pro-systems from $F$ to $G$ is the same thing as a transformation of functors $t : B \to A$. Namely, given $t$ we can apply $t$ to the class of $\text{id}_{G(j)}$ in $B(G(j))$ to get a compatible system of elements $\xi _ j \in A(G(j)) = \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(F(i), G(j))$ which is exactly our definition of a morphism in $\text{Pro-}\mathcal{C}$ in Remark 4.22.5. We omit the construction of a transformation $B \to A$ given a morphism of pro-objects from $F$ to $G$.

Lemma 4.22.8. Let $\mathcal{C}$ be a category. Let $M : \mathcal{I} \to \mathcal{C}$ be a diagram with filtered (resp. cofiltered) index category $\mathcal{I}$. Let $F : \mathcal{C} \to \mathcal{D}$ be a functor. If $M$ is essentially constant as an ind-object (resp. pro-object), then so is $F \circ M : \mathcal{I} \to \mathcal{D}$.

Proof. If $X$ is a value for $M$, then it follows immediately from the definition that $F(X)$ is a value for $F \circ M$. $\square$

Lemma 4.22.9. Let $\mathcal{C}$ be a category. Let $M : \mathcal{I} \to \mathcal{C}$ be a diagram with filtered index category $\mathcal{I}$. The following are equivalent

$M$ is an essentially constant ind-object, and
$X = \mathop{\mathrm{colim}}\nolimits _ i M_ i$ exists and for any $W$ in $\mathcal{C}$ the map

\[ \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, M_ i) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, X) \]

is bijective.

Proof. Assume (2) holds. Then $\text{id}_ X \in \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, X)$ comes from a morphism $X \to M_ i$ for some $i$, i.e., $X \to M_ i \to X$ is the identity. Then both maps

\[ \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, X) \longrightarrow \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, M_ i) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, X) \]

are bijective for all $W$ where the first one is induced by the morphism $X \to M_ i$ we found above, and the composition is the identity. This means that the composition

\[ \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, M_ i) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, X) \longrightarrow \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(W, M_ i) \]

is the identity too. Setting $W = M_ j$ and starting with $\text{id}_{M_ j}$ in the colimit, we see that $M_ j \to X \to M_ i \to M_ k$ is equal to $M_ j \to M_ k$ for some $k$ large enough. This proves (1) holds. The proof of (1) $\Rightarrow $ (2) is omitted. $\square$

Lemma 4.22.10. Let $\mathcal{C}$ be a category. Let $M : \mathcal{I} \to \mathcal{C}$ be a diagram with cofiltered index category $\mathcal{I}$. The following are equivalent

$M$ is an essentially constant pro-object, and
$X = \mathop{\mathrm{lim}}\nolimits _ i M_ i$ exists and for any $W$ in $\mathcal{C}$ the map

\[ \mathop{\mathrm{colim}}\nolimits _{i \in \mathcal{I}^{opp}} \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(M_ i, W) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, W) \]

is bijective.

Proof. Assume (2) holds. Then $\text{id}_ X \in \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, X)$ comes from a morphism $M_ i \to X$ for some $i$, i.e., $X \to M_ i \to X$ is the identity. Then both maps

\[ \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, W) \longrightarrow \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(M_ i, W) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, W) \]

are bijective for all $W$ where the first one is induced by the morphism $M_ i \to X$ we found above, and the composition is the identity. This means that the composition

\[ \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(M_ i, W) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, W) \longrightarrow \mathop{\mathrm{colim}}\nolimits _ i \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(M_ i, W) \]

is the identity too. Setting $W = M_ j$ and starting with $\text{id}_{M_ j}$ in the colimit, we see that $M_ k \to M_ i \to X \to M_ j$ is equal to $M_ k \to M_ j$ for some $k$ large enough. This proves (1) holds. The proof of (1) $\Rightarrow $ (2) is omitted. $\square$

Lemma 4.22.11. Let $\mathcal{C}$ be a category. Let $H : \mathcal{I} \to \mathcal{J}$ be a functor of filtered index categories. If $H$ is cofinal, then any diagram $M : \mathcal{J} \to \mathcal{C}$ is essentially constant if and only if $M \circ H$ is essentially constant.

Proof. This follows formally from Lemmas 4.22.9 and 4.17.2. $\square$

Lemma 4.22.12. Let $\mathcal{I}$ and $\mathcal{J}$ be filtered categories and denote $p : \mathcal{I} \times \mathcal{J} \to \mathcal{J}$ the projection. Then $\mathcal{I} \times \mathcal{J}$ is filtered and a diagram $M : \mathcal{J} \to \mathcal{C}$ is essentially constant if and only if $M \circ p : \mathcal{I} \times \mathcal{J} \to \mathcal{C}$ is essentially constant.

Proof. We omit the verification that $\mathcal{I} \times \mathcal{J}$ is filtered. The equivalence follows from Lemma 4.22.11 because $p$ is cofinal (verification omitted). $\square$

Lemma 4.22.13. Let $\mathcal{C}$ be a category. Let $H : \mathcal{I} \to \mathcal{J}$ be a functor of cofiltered index categories. If $H$ is initial, then any diagram $M : \mathcal{J} \to \mathcal{C}$ is essentially constant if and only if $M \circ H$ is essentially constant.

Proof. This follows formally from Lemmas 4.22.10, 4.17.4, 4.17.2, and the fact that if $\mathcal{I}$ is initial in $\mathcal{J}$, then $\mathcal{I}^{opp}$ is cofinal in $\mathcal{J}^{opp}$. $\square$

Comments (9)

Comment #538 by Nuno on April 08, 2014 at 01:55

Minor typos: (i) "Then $X$ comes equipped with projection morphisms $M_i \to X$ ." I believe projection should be injection here. (ii) "essentially constant as an pro-object".

Comment #549 by Johan on April 09, 2014 at 00:35

Thanks, fixed here.

Comment #4831 by Weixiao Lu on December 13, 2019 at 22:01

I think it should be mentioned in Definition 05PU that $X = \varinjlim_{i\in\mathcal I}M_i$ can be derived from conditions (a) and (b).

Comment #4837 by Johan on December 16, 2019 at 09:28

@#4831. Parts (a) and (b) do not make sense if you do not already know that $X = \lim M_i$ because we are using the coprojections $M_i \to X$ to formulate them. Moreover, I wanted to stress that given an essentially constant system, we have a colimit of the system with expected value.

Comment #5544 by Ingo Blechschmidt on October 19, 2020 at 20:55

But it is enough to suppose that $X$ is a (co-)cone of the diagram, and this observation is actually important in Lemma 05SH, as this lemma is (rightly) formulated without any exactness conditions on $F$ . Such conditions are not needed because any functor preserves (co-)cones even if it does not preserve (co-)limits.

Comment #5731 by Johan on November 16, 2020 at 13:19

@#4831 @#5544 OK, I now changed this a tiny bit. Thanks. Changes are in this commit.

Comment #6723 by Alejandro González Nevado on November 14, 2021 at 01:49

I think that the paragraph after Definition 05PV (although correct) is a bit confusing and should be divided into two different paragraphs because it seems to treat two different things that may be mixed by the reader. Maybe it is better to tag the first two sentences separately from the block with the example and the non-example (that seem to be not really directly connected to such fact) and write this last block with examples first instead. I say this because the example (and the non-example) refers of the definition above and it is not intended directly as an example of the fact mentioned with the first two sentences. I would suggest the following structure:

" Definition 05PV

(Here should be the example and the non-example)

If $(M_i,f_{ii′})$ is an essentially constant system and the morphisms f_{ii′} are monomorphisms, then for all $i\leq i′$ sufficiently large the morphisms f_{ii′} are isomorphisms. However, as we just saw in the example above, in general this need not be the case. "

I would personally find the suggested redistribution of the sentences in the referred paragraph much more clear in the text.

Comment #6918 by Johan on January 19, 2022 at 12:01

Thanks. OK, I tried to improve the text. See changes here. Also, I just noticed that maybe the alphabetical ordering in the contributors file is wrong for Spanish names. Is it?

Comment #8394 by Elías Guisado on May 14, 2023 at 10:16

I think it would be interesting to add the following remark to this section (maybe placing it after Definition 4.22.1): "Let $M:\mathcal{I}\to\mathcal{C}$ be a diagram, with $\mathcal{I}$ filtered (resp., cofiltered). Suppose that $M$ is essentially constant with value $X$ with respect to a morphism $X\to M_i$ (resp., to a morphism $M_i\to X$ ). Then, for any morphism $i\to j$ (resp., $j\to i$ ) in $\mathcal{I}$ , the diagram $M$ is also essentially constant with value $X$ with respect to the composite morphism $X\to M_i\to M_j$ (resp., $M_j\to M_i\to X$ ). Verifying this fact is an easy exercise that uses the (co)filteredness of $\mathcal{I}$ and we leave it to the reader."

The Stacks project

4.22 Essentially constant systems

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