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VojtechStep committed Dec 12, 2023
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Expand Up @@ -52,6 +52,9 @@ obtain a cocone

which is again a sequential colimit.

The result may be read as
`colimₙ (Σ (a : Aₙ) P(iₙ a)) ≃ Σ (a : colimₙ Aₙ) P(a)`.

## Definitions

### The sequential diagram for the flattening lemma
Expand Down Expand Up @@ -116,21 +119,21 @@ so it suffices to invoke the flattening lemma for coequalizers.
**Proof:** The diagram we construct is

```text
--------->
Σ ((n, a) : Σ ℕ A) P(iₙ a) ---------> Σ ((n, a) : Σ ℕ A) P(iₙ a) -----> Σ (x : X) P(x)
| | |
assoc-Σ | ≃ assoc-Σ | ≃ id | ≃
| | |
V ----------> V V
Σ (n :, a : Aₙ) P(iₙ a) ----------> Σ (n : ℕ, a : Aₙ) P(iₙ a) ------> Σ (x : X) P(x) ,
------->
Σ (n : ℕ) Σ (a : Aₙ) P(iₙ a) -------> Σ (n : ℕ) Σ (a : Aₙ) P(iₙ a) ----> Σ (x : X) P(x)
| | |
inv-assoc-Σ | ≃ inv-assoc-Σ | ≃ id | ≃
| | |
V ---------> V V
Σ ((n, a) : Σ ℕ A) P(iₙ a) ---------> Σ ((n, a) : Σ ℕ A) P(iₙ a) -----> Σ (x : X) P(x) ,
```

where the top is the cofork obtained by coforkification of the cocone for the
flattening lemma, and the bottom is the cofork obtained by flattening the
coforkification of the original cocone.
where the top is the cofork corresponding to the cocone for the flattening
lemma, and the bottom is the cofork obtained by flattening the cofork
corresponding to the given base cocone.

By assumption, the original cocone is a sequential colimit, which implies that
its coforkification is a coequalizer. The flattening lemma for coequalizers
its corresponding cofork is a coequalizer. The flattening lemma for coequalizers
implies that the bottom cofork is a coequalizer, which in turn implies that the
top cofork is a coequalizer, hence the flattening of the original cocone is a
sequential colimit.
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