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Algebraic geometry, Oslo 1970; proceedings by F. Oort

By F. Oort

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2 (2)). 2. Proposition. For any map f : L → K, the following triangle commutes: fk F un(L, C) ss s colimsL s sss 6 C G F un(K, C) u uu colimK u u uz uu Proof. Let F : L → C be a diagram. 2 (1), colimL F = colimcolimK df F = colimK colimdf F = colimK f k F . 3. Proposition. Let f : L → K be a map of spaces. Then the pull-back process f ∗ : F un(K, C) → F un(L, C) is right adjoint to the left Kan extension functor f k : F un(L, C) → F un(K, C). 4. Corollary. The left Kan extension f k : F un(L, C) → F un(K, C) commutes with colimits.

Assume that σ contains the vertex i. Let k be such that lk = i. We consider two cases. , lk+1 = i+1. In this case (di ◦ si )(σ) = (lm > · · · > lk+2 > i = i > · · · > l0 ) ∈ (∆[n + 1])m , and hence (di ◦ si )(σ) = sk dk+1 σ. Since si (σ) ∈ ∆[n] is of the form sk τ , the morphisms F (dk ) : F (dk σ) → F (σ) and F (dk+1 ) : F (dk+1 σ) → F (σ) are isomorphisms (F is si -bounded). We define s∗i F (σ) → F (σ) to be the composite: s∗i F (σ) = F (di ◦ si )(σ) = F (sk dk+1 σ) F (sk ) G F (dk+1 σ) F (dk+1 ) G F (σ) It is clear that this composite is an isomorphism as F (sk ) is so (F is a bounded diagram).

Let f : L → K be a map and F : L → M be an f -bounded diagram. Then F is f -cofibrant if and only if, for any simplex σ : ∆[n] → L such that f (σ) is non-degenerate in K, the morphism colim∂∆[n] F → F (σ) is a cofibration in M. The most significant aspect of being a relative cofibration is that this property can be checked locally. This is the key feature that absolute cofibrations are missing. Relative cofibrations have been introduced to enlarge the class of absolute cofibrations so the notion of cofibrancy would become local.

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