The following theorem may be useful in establishing maximums and minimums for the case of functions of two variables.

More investigation is necessary. Maxima and minima of functions subject to constraints. Let us set ourselves the following problem: Let F x, y and G x, y be functions defined over some region R of the x-y plane.

See Figure 1. The solution set i.

Let us now consider the same problem in three variables. Let F x, y, z and G x, y, z be functions defined over some region R of space. The problem then is to find the maximums of the function F x, y, z as evaluated on this spheroidal surface. Let us now consider another problem. Suppose instead of one side condition we have two. Let F x, y, z , G x, y, z and H x, y, z be functions defined over some region R of space.

Find the points at which the function F x, y, z has maximums subject to the side conditions.

## Morse : The critical points of functions and the calculus of variations in the large

Here we wish to find the maximum values of F x, y, z on that set of points that satisfy both equations 2 and 3. In Fig.

The intersection of the ellipsoid and the plane is the set F on which F x, y, z is to be evaluated. The above can be generalized to functions of n variables F x 1 , x 2 , Methods for finding maxima and minima of functions subject to constraints. Method of direct elimination. We can then substitute g x for y in F x, y and then find the maximums and minimums of F x, g x by standard methods. In some cases, it may be possible to do this kind of thing. As noted in Chapter 3, in multivariable calculus, the notion of differentiation manifests itself in several forms.

The simplest among these are the partial derivatives, which together constitute the gradient.

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When the gradient exists, its vanishing turns out to be a necessary condition for a function to have a local extremum. We shall use this in Section 4. A variant of the optimization problem discussed in the first section will be considered in Section 4.

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Such problems are nicely and effectively handled by a technique known as the method of Lagrange multipliers. The theoretical as well as practical aspects of this method will be discussed here. What magnitude will the maximum increase of temperature have? Theorem - Chain rule. The chain rule for the composition of a vectorial function with a scalar field allow us to get the algebra of derivatives for one-variable functions easily:. Applying the chain rule we get. Definition - Directional derivative. Remark : The partial derivatives are the directional derivatives along the vectors of the canonical basis.

Theorem - Implicit derivation. It can also be written as. As the partial derivatives of a function are also functions of several variables we can differentiate partially each of them. Definition - Hessian matrix. This fact is due to the following result.

## How do you find critical points for function of two variables #f(x,y)=8x^3+144xy+8y^3#?

As a consequence, if the function satisfies the requirements of the theorem for all the second order partial derivatives, the Hessian matrix is symmetric. In a previous chapter we saw how to approximate a one-variable function with a Taylor polynomial. This can be generalized to several-variables functions. Not all the critical points of a scalar field are points where the scalar field has relative extrema.