boundary — Mathlib

English

The boundary of M is the set of its boundary points: boundary M = { x ∈ M | I.IsBoundaryPoint x }.

Русский

Граница M — это множество его граничных точек: boundary M = { x ∈ M | I.IsBoundaryPoint x }.

LaTeX

$$$$ \mathrm{boundary} M = \{ x \in M \mid I.IsBoundaryPoint x \} $$$$

Lean4

/-- The **boundary** of a manifold `M` is the set of its boundary points. -/
protected def boundary : Set M :=
  {x : M | I.IsBoundaryPoint x}

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