ⅆ3⁡wⅆ⁡z3+3⁢f⁢ⅆ2⁡wⅆ⁡z2+(2⁢f2+f′+4⁢g)⁢ⅆ⁡wⅆ⁡z+(4⁢f⁢g+2⁢g′)⁢w=0.