
## 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 exists an $i \in \mathcal{I}$ and a morphism $X \to M_ i$ such that the composition $X \to M_ i \to X$ is $\text{id}_ X$. A stronger condition is the following.

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

1. Assume the index category $\mathcal{I}$ is filtered. We say $M$ is essentially constant with value $X$ if $X = \mathop{\mathrm{colim}}\nolimits _ i M_ i$ and there exists 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 $M_ j \to X \to M_ i \to M_ k$.

2. Assume the index category $\mathcal{I}$ is cofiltered. We say $M$ is essentially constant with value $X$ if $X = \mathop{\mathrm{lim}}\nolimits _ i M_ i$ and there exists 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 $M_ k \to M_ i \to X \to M_ j$.

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 4.22.3 and 4.22.4. 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. In general this need not be the case however. An example is 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 $\mathbf{Z}$. A non-example is to 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.

Remark 4.22.3. 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

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

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 $F$ 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 4.22.4. 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

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

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 $F$ is isomorphic to a constant system. If we ever need this material, then we will formulate this into a lemma and prove it here.

Lemma 4.22.5. 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 4.22.6. 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

1. $M$ is an essentially constant ind-object, and

2. $X = \mathop{\mathrm{colim}}\nolimits _ i M_ i$ exists and for any $W$ in $\mathcal{C}$ the map

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

is bijective.

Proof. Assume (2) holds. Then $\text{id}_ X \in \mathop{Mor}\nolimits _\mathcal {C}(X, X)$ 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

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

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

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

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 4.22.7. 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

1. $M$ is an essentially constant pro-object, and

2. $X = \mathop{\mathrm{lim}}\nolimits _ i M_ i$ exists and for any $W$ in $\mathcal{C}$ the map

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

is bijective.

Proof. Assume (2) holds. Then $\text{id}_ X \in \mathop{Mor}\nolimits _\mathcal {C}(X, X)$ 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

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

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

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

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 4.22.8. 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 4.22.6 and 4.17.2. $\square$

Lemma 4.22.9. 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 4.22.8 because $p$ is cofinal (verification omitted). $\square$

Lemma 4.22.10. 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 4.22.7, 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 $\mathcal{J}^{opp}$. $\square$

Comment #538 by Nuno on

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".

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