Associativity of pullbacks #
This file shows that pullbacks (and pushouts) are associative up to natural isomorphism.
(Xā Ć[Yā] Xā) Ć[Yā] Xā is the pullback (Xā Ć[Yā] Xā) Ć[Xā] (Xā Ć[Yā] Xā).
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(Xā Ć[Yā] Xā) Ć[Yā] Xā is the pullback Xā Ć[Yā] (Xā Ć[Yā] Xā).
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Xā Ć[Yā] (Xā Ć[Yā] Xā) is the pullback (Xā Ć[Yā] Xā) Ć[Xā] (Xā Ć[Yā] Xā).
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Xā Ć[Yā] (Xā Ć[Yā] Xā) is the pullback (Xā Ć[Yā] Xā) Ć[Yā] Xā.
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The canonical isomorphism (Xā Ć[Yā] Xā) Ć[Yā] Xā ā
Xā Ć[Yā] (Xā Ć[Yā] Xā).
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(Xā ⨿[Zā] Xā) ⨿[Zā] Xā is the pushout (Xā ⨿[Zā] Xā) Ć[Xā] (Xā ⨿[Zā] Xā).
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(Xā ⨿[Zā] Xā) ⨿[Zā] Xā is the pushout Xā ⨿[Zā] (Xā ⨿[Zā] Xā).
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Xā ⨿[Zā] (Xā ⨿[Zā] Xā) is the pushout (Xā ⨿[Zā] Xā) Ć[Xā] (Xā ⨿[Zā] Xā).
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Xā ⨿[Zā] (Xā ⨿[Zā] Xā) is the pushout (Xā ⨿[Zā] Xā) ⨿[Zā] Xā.
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The canonical isomorphism (Xā ⨿[Zā] Xā) ⨿[Zā] Xā ā
Xā ⨿[Zā] (Xā ⨿[Zā] Xā).