Baroclinic growth doubling: σ(2U)/σ(U) = 2

Sympy-exact witness of the baroclinic-instability shear-doubling identity. Setup: σ = U·f/(H·N) as a four-symbol function. Identity: σ(2U)/σ(U) = (2U·f/(H·N)) / (U·f/(H·N)) = 2 — exact-linear doubling in U. Oracle pins: sigma_sym =…