\[\frac{{\mathrm{d}}^{3}w}{{\mathrm{d}z}^{3}}+3f\frac{{\mathrm{d}}^{2}w}{{%
\mathrm{d}z}^{2}}+(2f^{2}+f^{\prime}+4g)\frac{\mathrm{d}w}{\mathrm{d}z}+(4fg+2%
g^{\prime})w=0.\]