# Calculus of variations: Lagrange multipliers

Given a functional $$J(y)=\int_a^b F(x,y,y')dx, \tag{1}$$
where $y$ is a function of $x$, and a constraint $$\int_a^b K(x,y,y')dx=l, \tag{2}$$ if $y=y(x)$ is an extreme of (1) under the constraint (2), then there exists a constant $\lambda$ such that $y=y(x)$ is also an extreme of the functional $$\int_a^b [F(x,y,y')+\lambda K(x,y,y')]dx. \tag{3}$$ Similarly, if the constraint is $$g(x,y,y')=0, \tag{4}$$ then there exists a function $\lambda(x)$ such that the extreme also holds for the functional $$\int_a^b [F(x,y,y')+\lambda(x)g(x,y,y')]dx. \tag{5}$$

This is known as the Lagrange multiplier rule for calculus of variations. However, I have two questions about this statement.

1. If the functional (1) has two constraints (2) and (4), does the extreme also hold for the functional $$\int_a^b [F(x,y,y')+\lambda K(x,y,y') +\lambda(x) g(x,y,y')]dx ? \tag{6}$$
2. Is this statement also valid for multiple variable case? For example, if $J=\iint F(x_1,x_2,y(x_1,x_2))dx_1 dx_2$, and $\iint K(x_1,x_2,y(x_1,x_2))dx_1 dx_2=l$, is this equivalent to $J=\iint F(x_1,x_2,y(x_1,x_2))+\lambda K(x_1,x_2,y(x_1,x_2))dx_1 dx_2$?

Thanks in advance and any suggestion will be appreciated. It is better if you have any reference.

The short answer is yes and yes. As a reference, I would recommend (highly) an inexpensive general math reference that I have found helpful: Mathematical Handbook for Scientists and Engineers. It has practically every applied math theorem and technique you'd want. Note that it is a math reference, not a computational/engineering reference, so it discusses the theoretical aspects. I found answers to your questions on pages 11.6-3 and 11.6-9, respectively. The general Lagrange multiplier $\lambda(x)$ has, as a specific case the constant function $\lambda_{i}$ for an isoparametric constraint of the type you indicated in (2).
• Yes, that helps. Actually, I am working on a functional $$J(y_1,y_2)=\iint F(x_1,x_2,y_1(x_1),y_2(x_2))dx_1 dx_2$$ and its Lagrange multipliers w.r.t constraint such as $$\iint K(x_1,x_2,y(x_1),y(x_2))dx_1 dx_2=l.$$ Note that in my case, each fucntion only depends on one variable. Unfortunately, I have not seen any textbook regarding this. Do you have any reference suggestions for this kind of problem. Many thanks. – MIMIGA Oct 17 '13 at 23:52
• That shouldn't be an issue, as you can represent each single-variable function as a degenerate two-variable function: $y_1(x_1) = y^{'}_{1}(x_1,x_2) = y_1(x_1)+0x_2$. A similar arugment can be made for $y_2(x_2)$. Just do that modification and carry though the euler-lagrange equations...you will see that making each y a multi-variable function will have no effect, as the derivatives wrt the ancillary variable will be zero. – user76844 Oct 18 '13 at 12:55
• You mean the euler-lagrange still holds like $$\frac{\partial{K}}{\partial{y_1}}=0, ~\frac{\partial{K}}{\partial{y_2}}=0.$$ Actually, that is not true. You can refer to my previous post. Thanks. math.stackexchange.com/questions/520761/… – MIMIGA Oct 18 '13 at 14:42
• Example: for $y_1: \frac{d}{dx_1}(\frac{\partial K}{\partial y_{1|x_2}})+\frac{d}{dx_2}(\frac{\partial K}{\partial y_{1|x_1}}) - \frac{\partial K}{\partial y_1} = 0$. The second term will be zero, as $y_1$ is not sensitive to changes in $x_2$ – user76844 Oct 18 '13 at 14:56