Chapter Two Fermat: Two or More Variables without Constraints Near a maximum the changes [· · · ] are in the beginning only im- perceptible. J. Kepler · How do we find the extrema of a function of two or more vari- ables without constraints, that is, how can we solve a problem f (x) extr, where f is a function of x 1 , . . . , x n ? 2.0 SUMMARY Two or more variables of optimization. Sometimes an optimal