  We analyze the response of bilayer graphene to an external transverse
electric field using a variational method. A previous attempt to do so in a
recent paper by Falkovsky [Phys. Rev. B 80, 113413 (2009)] is shown to be
flawed. Our calculation reaffirms the original results obtained by one of us
[E. McCann, Phys. Rev. B 74, 161403(R) (2006)] by a different method. Finally,
we generalize these original results to describe a dual-gated bilayer graphene
device.
