Is this statement meaningful if one of the elements is undefined? Am I allowed to say a statement like $\max\left\lbrace a,b\right\rbrace$ if it turns out that the element $b$ is undefined, or simply does not exist?  Would the result be $a$, or is the whole statement invalid?
edit: based on the answers I've seen, I think I will have to break my proof into separate cases.  I always want to avoid that when possible, but I would rather make sense in what I write.
 A: Hint: $$\max\left\lbrace a,b\right\rbrace=\dfrac{a+b+|a-b|}{2}$$ 

 If you don't know $b$ then you can't determine $|a-b|$ and the rest of the formula ;)

A: The $\text{max}$ operator requires comparison among arguments, or alternatively, requires that the arguments have values which can be used to compute $\operatorname{max}\{a, b\} = \dfrac{a+b+|a-b|}2$.  Since we cannot compare an incomparable (since it is undefined) number $b$ with any other number(s), operating on them is rendered meaningless.
It is analogous to asking what $f(x) = x^2$ when evaluated at the undefined value of $b$.
A: You're "allowed" to say what you like, as long as you can define what it means.
If it's convenient for your proof to define some quantity that's equal to the maximum of $a$ and $b$ except in the cases where $b$ is undefined, in which case it's equal to $a$, then go right ahead.
The only remaining issue is whether you refer to this quantity as $\max\{a,b\}$, or give it some other name. If you're going to use properties of $\max$ that aren't clearly true of your quantity, then it would be wise to give it a different name. For example using $\max\{a,b\} \ge a$ won't cause any problems. Using $\max\{a,b\} \ge b$ might lead you to make errors, since it's not true (or meaningful even) in the cases for which $b$ is undefined.
A: If one of the elements is $\infty$ or $-\infty$ (as a limit), then you can extend the definition of $max\{a, b\}$ to be meaningful. I can't think of any other instance where it would make sense to use an argument that you would call undefined.
