theorem cross_coupling_creates_spiral
{a d b c : ℝ}
-- Diagonal entries negative (self-correcting)
(ha : a < 0) (hd : d < 0)
-- Cross-coupling strong enough: 4(ad - bc) > (a+d)²
(hstrong : (a + d) ^ 2 < 4 * (a * d - b * c)) :
jacobianDiscriminant (jacobianTrace a d) (jacobianDet a d b c) < 0 := by
simp only [jacobianDiscriminant, jacobianTrace, jacobianDet]
linarithCoupled (ρ, T) Jacobian Analysis