Theorem: psi'(x) + psi(x)/delta_W = 0 (Stommel boundary-layer exponential decay)

Theorem (Stommel-boundary-ODE canonical): for psi(x) = psi_0 exp(-x/delta_W), differentiating gives psi'(x) = -psi_0/delta_W exp(-x/delta_W) = -psi/delta_W. Hence psi'(x) + psi(x)/delta_W = 0 identically. Canonical sympy pin: psi_0, x,…