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theorem proved term proof

deriv_coordRay_j

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formal statement (Lean)

 388theorem deriv_coordRay_j (x : Fin 4 → ℝ) (i j : Fin 4) (h : j ≠ i) :
 389    deriv (fun t => (coordRay x i t) j) 0 = 0 := by

proof body

Term-mode proof.

 390  simp only [coordRay_apply, basisVec_ne h, mul_zero, add_zero]
 391  exact deriv_const 0 (x j)
 392
 393/-- **THEOREM**: Functional derivative of spatialNormSq.
 394    ∂_μ (∑ x_i²) = 2 x_μ for μ ∈ {1,2,3}, else 0.
 395
 396    **Derivation**: Using the chain rule and ∂_μ(x_i²) = 2x_i δ_{iμ}, we get:
 397    ∂_μ(x₁² + x₂² + x₃²) = 2x₁δ_{1μ} + 2x₂δ_{2μ} + 2x₃δ_{3μ} = 2x_μ for μ ∈ {1,2,3}. -/

depends on (14)

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