Theorem 88: the chart missing the strict transform of the hypersurface #
In the proof of [Kol07, Theorem 88], only r − 1 of the charts of the blow-up have the displayed
form, but "these r − 1 charts, however, completely cover S_1": the chart whose distinguished
coordinate is
the equation y_ρ of S misses the strict transform S_1. In the chart description of the stalk
of the blow-up at a point q ∈ B_Z X over z ∈ Z with S_z = (y_ρ), the stalk of S_1 at q is
the saturation ⋃ᵢ (π^* S_z : F_qⁱ) of the total transform by the exceptional ideal
(stalkIdeal_saturate_of_isInvertible), where π^* S_z = (π^♯ y_ρ) = F_q
(stalkIdeal_comap_eq_span_of_chart); so 1 ∈ (F_q : F_q) and (S_1)_q = 𝒪_{B,q}. This is the
observation in the proof of [Kol07, Lemma 62] that the remaining chart "does not contain any point
of the birational transform".
In the chart description of the stalk at q ∈ B_Z X over z ∈ Z, when the distinguished
coordinate y_ρ is the local equation of S (S_z = (y_ρ)), the strict transform of S misses
q: (S_1)_q = 𝒪_{B,q} (the proofs of [Kol07, Lemma 62 and Theorem 88]).