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Is every set a filtered colimit of finite sets?

2

$begingroup$

Is the following statement correct in the category of sets?

Let $X$ be any set. Then there exists a filtered small category $I$ and a functor $F:Ito mathrmSet$ such that for all $iin I$ the set $F(i)$ is finite, and such that
$$
X ; = ; mathrmcolim_iin I F(i) .
$$

Are there references on results of this type in the literature?

reference-request category-theory limits-colimits

share|cite|improve this question

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

asked Apr 8 at 12:29

geodudegeodude

4,1911344

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  One way to generalize this is the notion of a locally finitely presentable category.
  $endgroup$
  – Derek Elkins
  Apr 8 at 12:35
  

  
  
  
  

add a comment | 

2

$begingroup$

Is the following statement correct in the category of sets?

Let $X$ be any set. Then there exists a filtered small category $I$ and a functor $F:Ito mathrmSet$ such that for all $iin I$ the set $F(i)$ is finite, and such that
$$
X ; = ; mathrmcolim_iin I F(i) .
$$

Are there references on results of this type in the literature?

reference-request category-theory limits-colimits

share|cite|improve this question

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

asked Apr 8 at 12:29

geodudegeodude

4,1911344

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  One way to generalize this is the notion of a locally finitely presentable category.
  $endgroup$
  – Derek Elkins
  Apr 8 at 12:35
  

  
  
  
  

add a comment | 

$begingroup$

Is the following statement correct in the category of sets?

Let $X$ be any set. Then there exists a filtered small category $I$ and a functor $F:Ito mathrmSet$ such that for all $iin I$ the set $F(i)$ is finite, and such that
$$
X ; = ; mathrmcolim_iin I F(i) .
$$

Are there references on results of this type in the literature?

reference-request category-theory limits-colimits

share|cite|improve this question

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

asked Apr 8 at 12:29

geodudegeodude

4,1911344

$endgroup$

share|cite|improve this question

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

asked Apr 8 at 12:29

geodudegeodude

4,1911344

share|cite|improve this question

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

asked Apr 8 at 12:29

geodudegeodude

4,1911344

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

edited Apr 8 at 19:10

Andrés E. Caicedo

66k8160252

asked Apr 8 at 12:29

geodudegeodude

4,1911344

asked Apr 8 at 12:29

geodudegeodude

4,1911344

2 Answers
2

active

oldest

votes

13

$begingroup$

The answer is yes: every set is the union of its finite subsets.

So take $I = P_textfinite(X)$ with as morphisms the inclusion maps, and $F : I to textSet$ the inclusion.

share|cite|improve this answer

answered Apr 8 at 12:31

rabotarabota

14.6k32886

$endgroup$

add a comment | 

9

$begingroup$

One answer already mentions the diagram of finite subsets of $X$. You would have to check that taking the union of this system actually is the colimit (which is an easy exercise).

Since you asked for a reference, Locally Presentable and Accessible Categories by J. Adámek and J. Rosický is a great book on this kind of stuff. In particular example 1.2(1) already mentions the diagram of finite subsets.

share|cite|improve this answer

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616

New contributor

Mark Kamsma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

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13

$begingroup$

The answer is yes: every set is the union of its finite subsets.

So take $I = P_textfinite(X)$ with as morphisms the inclusion maps, and $F : I to textSet$ the inclusion.

share|cite|improve this answer

answered Apr 8 at 12:31

rabotarabota

14.6k32886

$endgroup$

add a comment | 

13

$begingroup$

The answer is yes: every set is the union of its finite subsets.

So take $I = P_textfinite(X)$ with as morphisms the inclusion maps, and $F : I to textSet$ the inclusion.

share|cite|improve this answer

answered Apr 8 at 12:31

rabotarabota

14.6k32886

$endgroup$

add a comment | 

$begingroup$

The answer is yes: every set is the union of its finite subsets.

So take $I = P_textfinite(X)$ with as morphisms the inclusion maps, and $F : I to textSet$ the inclusion.

share|cite|improve this answer

answered Apr 8 at 12:31

rabotarabota

14.6k32886

$endgroup$

share|cite|improve this answer

answered Apr 8 at 12:31

rabotarabota

14.6k32886

answered Apr 8 at 12:31

rabotarabota

14.6k32886

answered Apr 8 at 12:31

rabotarabota

14.6k32886

9

$begingroup$

One answer already mentions the diagram of finite subsets of $X$. You would have to check that taking the union of this system actually is the colimit (which is an easy exercise).

Since you asked for a reference, Locally Presentable and Accessible Categories by J. Adámek and J. Rosický is a great book on this kind of stuff. In particular example 1.2(1) already mentions the diagram of finite subsets.

share|cite|improve this answer

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616

New contributor

Mark Kamsma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

9

$begingroup$

One answer already mentions the diagram of finite subsets of $X$. You would have to check that taking the union of this system actually is the colimit (which is an easy exercise).

Since you asked for a reference, Locally Presentable and Accessible Categories by J. Adámek and J. Rosický is a great book on this kind of stuff. In particular example 1.2(1) already mentions the diagram of finite subsets.

share|cite|improve this answer

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616

New contributor

Mark Kamsma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

$begingroup$

One answer already mentions the diagram of finite subsets of $X$. You would have to check that taking the union of this system actually is the colimit (which is an easy exercise).

Since you asked for a reference, Locally Presentable and Accessible Categories by J. Adámek and J. Rosický is a great book on this kind of stuff. In particular example 1.2(1) already mentions the diagram of finite subsets.

share|cite|improve this answer

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616

New contributor

Mark Kamsma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|cite|improve this answer

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616

New contributor

Mark Kamsma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616

New contributor

Mark Kamsma is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered Apr 8 at 12:40

Mark KamsmaMark Kamsma

3616