You signed out in another tab or window. One proof: Now the composite g0F: X × I → Z is a homotopy between g0f0 and g0f1, and the composite G(f1 × idI): X × I → Z is a homotopy between g0f1 and g1fBy lemma 1 ChapterSkipped for triviality–3, 9–12,–15,,–22,