# Power series question.

1. Jan 1, 2013

### peripatein

Hi,
1. The problem statement, all variables and given/known data
I am asked to prove that if the power series Ʃ(1,infinity) a_n(x-x0)^n converges at a point d, then it converges for every x that satisfies |x-x0|<|d-x0|.

2. Relevant equations

3. The attempt at a solution
Obviously |d-x0|<r, where r denotes the radius of convergence. Should I strive to demonstrate that every such x would also satisfy |x-x0|<r? Is that the way to go about proving the above proposition? There is a general rule that if a power series converges at a point a, then it converges at every point x satisfying |x|<|a|. Should I therefore strive to show that |x|<|d|, or am I actually requested to prove the general rule? I'd appreciate some guidance.

2. Jan 1, 2013

### haruspex

It seems to me you are being asked to prove that there is such a thing as radius of convergence, so better not to assume it. But it is hard to be sure.

3. Jan 1, 2013

### peripatein

What would you suggest then?

4. Jan 1, 2013

### haruspex

If in doubt, go back to first principles. Given ε > 0, there exists... You should be able to use this criterion wrt x-x0 to show satisfied for some x closer x0.

5. Jan 1, 2013

### peripatein

So what keeps me from letting epsilon be the radius itself?

6. Jan 1, 2013

### haruspex

ε would be a bound in relation to the value to which the series converges, not in relation to x. Btw I should have said 'use the fact that this criterion is satisfied wrt d-x0 ...'.