# At what point do the sums of the reciprocals converge?

1. Aug 17, 2011

### thegreatjared

It is well known that the Harmonic Series diverges (1/1+1/2+1/3+1/4+...), but that the series [1/1+1/4+1/9+1/16+...] converges. In the first series, the denominators are the integers, whereas in the second example, the denominators are the integers to the power of 2. My question is, at what power do the sums of the reciprocals "switch" from divergence to convergence?

2. Aug 17, 2011

### phyzguy

The integral test for convergence tells us that the infinite sum:
$$\sum_1^\infty f(n)$$
and the integral:
$$\int_1^\infty f(n)$$
either both converge or both diverge. Since:
$$\int_1^\infty \frac{1}{x^{1+\delta}} = \frac{1}{\delta}$$
This tells us that the infinite series:
$$\sum_1^\infty \frac{1}{x^{1+\delta}}$$
will converge as long as delta is greater than zero, no matter how small it is.

3. Aug 17, 2011

### thegreatjared

Wow, simple as that. Thanks!!

4. Aug 17, 2011

### micromass

Staff Emeritus