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theorem proved term proof

lightlike_iff_speed_c

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formal statement (Lean)

 202theorem lightlike_iff_speed_c (v : Displacement) :
 203    interval v = 0 ↔ spatial_norm_sq v = temporal_sq v := by

proof body

Term-mode proof.

 204  rw [interval_eq_spatial_minus_temporal]; constructor <;> intro h <;> linarith
 205
 206/-- **Timelike condition**: ds² < 0 iff |Δx|² < (Δt)². -/

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