ABSTRACT
If R is the function that minimizes s, then the derivative of s with respect to an infinitesimal change in R must be zero. To do this, replace R(x, y) with R(x, y)+eh(x) where e is any small positive number and h is any function of x other than h=0 everywhere. There could be a different h for each value of y, but it is not denoted explicitly. As we did in the §14.1, we shall use a Lagrange multiplier - 2? and write
In this equation, the term in square brackets will always be zero if the constraint (that A be unimodular) is fulfilled.
