Difficulties may arise when the problem becomes slightly more complicated. A seemingly modest twist on the parallel flow above would be the radial flow between parallel plates; this involves convection and thus non-linearity. The velocity field may be represented by a function f (z) that must satisfy: d 2 f d z 2 + R f 2 = − − 1 ; f ( − − 1 ) = f ( 1 ) = 0.