In fluid field, Bernard  described the equations of motion for six velocity potentials of perfect fluids which led to a variational principle
that reproduced the Eulerian equation of motion.
Li and Song demonstrate how variational methods can be used in nonlinear differential equations, but begin by reviewing prerequisites such as Sobolev space and the variational principle
for readers who are a little rusty.
1] which is a more precise version of Ricceri's Variational Principle
[25, Theorem 2.
We shall do this by combining and extending two recent developments: a recent reformulation of the enhanced sampling problem into a powerful variational principle
that opens a wealth of possibilities and provides a novel and fruitful standpoint for new developments; and a procedure for extracting rates from enhanced runs.
Thus, referring to reference  for applying least square method to establish "partial and temporary unified theory of natural science so far" including all the equations of natural science so far (in which, the theory of everything to express all of natural laws, described by Hawking that a single equation could be written on a T-shirt, is partially and temporarily realized in the form of "partial and temporary unified variational principle
of natural science so far"), Eq.
We generalize Wheeler-Feynman electrodynamics [1,2] with a variational principle
whose extrema are required to satisfy a boundary-value problem [3,4].
Its variational principle
can be established using the semi-inverse method [27, 28]:
From the reduced variational principle
, [delta] [integral] <L> dz = 0 and Eq.
Key words: vibrations, variational principle
, displacements fields, connecting rod
These statements include the maximality results in Isac  and Nemeth ; which, in turn, extend Ekeland's variational principle
 (in short: EVP); so, it is natural to denote their union as (EVPv).
3] or ) and Ekeland's Variational Principle
By using the semi-inverse method  the variational principle
of the Eq.