# Continuous function example

1. Mar 25, 2008

### gaborfk

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

For each $$a\in\mathbb{R}$$, find a function $$f$$ that is continuous at $$x=a$$ but discontinuous at all other points.

3. The attempt at a solution

I guess I am not getting the question. I need to come up with a function, I was thinking of a piecewise defined one, half rational half irrational, which is continuous on one but not the other? Is this possible?

2. Mar 25, 2008

### sutupidmath

what about f(x)={0, x-rational, x, where x irrational.
take a sequence {a} that converges to 0, from this sequence lets take two subsequences {b} of rationals, and {c} of irrationals, since {a} converges to 0 also {b} and {c} should converge to zero. now lets take the corresponding sequence of the function

f({a})-->0

f({b})-->x-->0

So this function i guess is continuous at x=0, since also f(0)=0, but it is discontinuous everywhere else.

Let's see what other guys have to say on this, cuz, i am not 100% sure that what i did actually works.

3. Mar 25, 2008

### gaborfk

Thank you!

That sound great.

4. Mar 25, 2008

### sutupidmath

Can you show why the function that i took as an example, from the top of my head, is everywhere else discontinous, because i left this part for you to show.???

5. Mar 25, 2008

### gaborfk

Because there are infinitely many irrational numbers which would make the graph continuous on the irrationals, but on an interval there would be rationals mixed in between the irrationals?

6. Mar 25, 2008

### sutupidmath

Well, try to use the same logic i used to show that it is continuous at 0. In other words try to use sequences and see if you can come up with sth. It is quite trivial frome here, i guess.