# Local max and min at a if lim f(x) approaching a does not exist

1. Nov 28, 2009

### merry

Hi,
I understand that the local max of a function is the point at which the y value of the function is larger than the neighbouring y values of the function.
Say we're considering the local max at a of a function f(x). Does the function have to exist on both sides of a for (a,f(a)) to be a local max? (consider the same situation for a local min).
In short, if the fuction exists at [a, b] $$\epsilon$$$$\Re$$ only,
can there be a local max or min at (a, f(a)) or (b, f(b)) ?
My high school professor said that to have local extrema, the function should exist on either side of the point. However, I believe my University prof said that this is not the case.
Could someone please clarify as to which one is the case?
Thanks a ton!
Merry

2. Nov 29, 2009

### mathman

It appears to be a quibble over definition. If you have a function defined over a finite interval, the end point values will almost always be local extrema (unless the function is constant near the end point). For pedogogical reasons, it is better to consider only interior points.

3. Dec 11, 2009

### merry

Thanks! =D
I asked my Uni prof and he said that our high school textbooks are wrong and that local extrema can exist at end points xD