In 1-D, the minimization problem can be stated as
Find such that
is a minimum.
We have seen that the minimization problem can be reduced down to the solution of an Euler equation
with the associated boundary conditions
or,
In 3-D, the equivalent minimization problem can be stated as
Find such that
is a minimum.
We would like to find the Euler equation for this problem and the associated boundary conditions required to minimize .
Let us define all our quantities with respect to an orthonormal basis .
Then,
and
Taking the first variation of , we get
All the nine components of are not independent. Why ?
The variation of the functional needs to be expressed
entirely in terms of . We do this using the 3-D equivalent of integration by parts - the divergence theorem.
Thus,
Substituting in the expression for , we have,
For to be minimum, a necessary condition is that for all variations .
Therefore, the Euler equation for this problem is
and the associated boundary conditions are