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Claim 15.1: the distance from x to the hyperplane {v : ⟨w,v⟩ + b = 0} with ‖w‖ = 1 is |⟨w,x⟩ + b|

Proved
UnderstandingML.dist_to_hyperplane

by naimengye · Sep 24, 2026 · Mathlib 0df444a (Lean v4.33.1)

geometrymargin

Claim 15.1. The distance between a point xxx and the hyperplane defined by (w,b)(w, b)(w,b) where ∥w∥=1\|w\| = 1∥w∥=1 is ∣⟨w,x⟩+b∣|\langle w, x\rangle + b|∣⟨w,x⟩+b∣.

Preamble
import Definitions.Def_UnderstandingML_SVM

open MeasureTheory
open scoped InnerProductSpace
Formal statement
namespace UnderstandingML

/-- **Claim 15.1** (p. 203). The distance between a point `x` and the hyperplane defined by
`(w, b)` where `‖w‖ = 1` is `|⟨w, x⟩ + b|`. -/
theorem dist_to_hyperplane {d : ℕ} (w : Vec d) (hw : ‖w‖ = 1) (b : ℝ) (x : Vec d) :
    Metric.infDist x {v : Vec d | ⟪w, v⟫_ℝ + b = 0} = |⟪w, x⟫_ℝ + b| := by sorry

end UnderstandingML
Source
Shalev-Shwartz and Ben-David, Understanding Machine Learning: From Theory to Algorithms, Cambridge University Press 2014, doi:10.1017/CBO9781107298019, §15.1 p. 203, Claim 15.1 with its proof
Human review
  • Endorsed by Shuze Chen · Sep 25, 2026

    Confirmed by the moderator at approval.

  • Endorsed by naimengye · Sep 25, 2026

    Confirmed by the mission captain (proposal self-audit).

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