  The determination of the speed of travelling fronts of the scalar reaction
diffusion equation has been the subject of much study. Using different
approaches seemingly disconnected variational principles have been established.
The purpose of this work is to show the connection between them. For monostable
reaction terms, we prove that a principle established by Hadeler and Rothe in
1975 and a second one by Benguria and Depassier in 1996 are logically
equivalent, that is, either can be derived from the other. Two variational
principles, formulated for arbitrary reaction terms, are shown to be related by
a suitable change of variables. Finally a variational principle proven for
monostable reaction terms is shown to be a formulation of the two previous ones
in yet another independent variable.
