Instability of Laminar Flow of a Perfect Magnetofluid

A simple sufficient condition is given for instability against small oscillations of symmetric laminar flows of an inviscid incompressible perfectly conducting magnetofluid, flowing between parallel walls in the presence of a symmetric magnetic field parallel to the flow. The condition is interesting because it shows that any such flow that is stable in the absence of a magnetic field can be made unstable by the superposition of a suitable magnetic field parallel to the flow.