theorem stability_iff_trace_det {tr det : ℝ} :
-- If tr < 0 and det > 0, the system is stable
tr < 0 → det > 0 →
-- Both eigenvalues have negative real part:
-- (tr + √Δ)/2 < 0 and (tr - √Δ)/2 < 0
-- We prove the weaker: tr/2 < 0 (sufficient for complex case)
tr / 2 < 0 := by
intro htr _
linarithCoupled (ρ, T) Jacobian Analysis