Discontinuous functions

1. Jan 23, 2010

Kate2010

1. The problem statement, all variables and given/known data

I have to find f: R $$\rightarrow$$ R which is discontinuous at the points of the set {1/n : n a positive integer}$$\cup$$ {0} but continuous everywhere else.

Also find g: R $$\rightarrow$$ R which is discontinuous at the points of the set {1/n : n a positive integer}but continuous everywhere else.

2. Relevant equations

3. The attempt at a solution

Could I define f as f(x) = 1/(integer(x) -1) for x $$\in$$ [0,1) (where integer means round up to next integer), f(x) = x otherwise.

Similarly for g, can I say g(x) = 1/(integer(x) -1) for x $$\in$$ (0,1) (where integer means round up to next integer), g(x) = x otherwise.

I'm not too sure about these functions but cannot think of any more 'normal' ones that would satisfy the criteria.

2. Jan 24, 2010

CompuChip

If you are looking for a 'normal' function, you probably want f(x) = 1/g(x), where g(x) has a zero at every integer n and at 0. In that case, I can think of a suitable g(x) off the top of my head.

By the way, note that restricting the domain to [0, 1) or (0, 1) does not define a function on R.

3. Jan 24, 2010

Kate2010

Can I say f(x) = 1/g(x) where:
g(x) = 0 if x is in (0,1)
g(x) = 1 otherwise

4. Jan 24, 2010

snipez90

Well no, since then you are having it be discontinuous on (0,1). Perhaps try 1/g, where g is a step function with an appropriate argument so that you get 1/x back when x is of the form 1/n, but the left and right limits as you approach an x of this form are different fractions altogether.

5. Jan 24, 2010

Kate2010

I think I may be confused about what 'at the points of the set' means, does it just mean the end points, I thought it meant all points in the set?

So for {1/n: n a positive integer} U {0} could I say:
f(x) = -1, x<0
f(x) = -1/2, x=0
f(x) = 1/x, 0<x<1
f(x) = 2, x>1

For {1/n: n a positive integer} I could change this to:
f(x) = -1, x$$\leq$$0
f(x) = 1/x, 0<x<1
f(x) = 2, x>1