Best guess for the function is
![\displaystyle f(x) = \sum_{n=1}^\infty \frac{x^n}{n^2}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20f%28x%29%20%3D%20%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20%5Cfrac%7Bx%5En%7D%7Bn%5E2%7D)
By the ratio test, the series converges for
![\displaystyle \lim_{n\to\infty} \left|\frac{x^{n+1}}{(n+1)^2} \cdot \frac{n^2}{x^n}\right| = |x| \lim_{n\to\infty} \frac{n^2}{(n+1)^2} = |x| < 1](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Clim_%7Bn%5Cto%5Cinfty%7D%20%5Cleft%7C%5Cfrac%7Bx%5E%7Bn%2B1%7D%7D%7B%28n%2B1%29%5E2%7D%20%5Ccdot%20%5Cfrac%7Bn%5E2%7D%7Bx%5En%7D%5Cright%7C%20%3D%20%7Cx%7C%20%5Clim_%7Bn%5Cto%5Cinfty%7D%20%5Cfrac%7Bn%5E2%7D%7B%28n%2B1%29%5E2%7D%20%3D%20%7Cx%7C%20%3C%201)
When
,
is a convergent
-series.
When
,
is a convergent alternating series.
So, the interval of convergence for
is the <em>closed</em> interval
.
The derivative of
is the series
![\displaystyle f'(x) = \sum_{n=1}^\infty \frac{nx^{n-1}}{n^2} = \frac1x \sum_{n=1}^\infty \frac{x^n}n](https://tex.z-dn.net/?f=%5Cdisplaystyle%20f%27%28x%29%20%3D%20%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20%5Cfrac%7Bnx%5E%7Bn-1%7D%7D%7Bn%5E2%7D%20%3D%20%5Cfrac1x%20%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20%5Cfrac%7Bx%5En%7Dn)
which also converges for
by the ratio test:
![\displaystyle \lim_{n\to\infty} \left|\frac{x^{n+1}}{n+1} \cdot \frac n{x^n}\right| = |x| \lim_{n\to\infty} \frac{n}{n+1} = |x| < 1](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Clim_%7Bn%5Cto%5Cinfty%7D%20%5Cleft%7C%5Cfrac%7Bx%5E%7Bn%2B1%7D%7D%7Bn%2B1%7D%20%5Ccdot%20%5Cfrac%20n%7Bx%5En%7D%5Cright%7C%20%3D%20%7Cx%7C%20%5Clim_%7Bn%5Cto%5Cinfty%7D%20%5Cfrac%7Bn%7D%7Bn%2B1%7D%20%3D%20%7Cx%7C%20%3C%201)
When
,
becomes the divergent harmonic series.
When
,
is a convergent alternating series.
The interval of convergence for
is then the <em>closed-open</em> interval
.
Differentiating
once more gives the series
![\displaystyle f''(x) = \sum_{n=1}^\infty \frac{n(n-1)x^{n-2}}{n^2} = \frac1{x^2} \sum_{n=1}^\infty \frac{(n-1)x^n}{n} = \frac1{x^2} \left(\sum_{n=1}^\infty x^n - \sum_{n=1}^\infty \frac{x^n}n\right)](https://tex.z-dn.net/?f=%5Cdisplaystyle%20f%27%27%28x%29%20%3D%20%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20%5Cfrac%7Bn%28n-1%29x%5E%7Bn-2%7D%7D%7Bn%5E2%7D%20%3D%20%5Cfrac1%7Bx%5E2%7D%20%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20%5Cfrac%7B%28n-1%29x%5En%7D%7Bn%7D%20%3D%20%5Cfrac1%7Bx%5E2%7D%20%5Cleft%28%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20x%5En%20-%20%5Csum_%7Bn%3D1%7D%5E%5Cinfty%20%5Cfrac%7Bx%5En%7Dn%5Cright%29)
The first series is geometric and converges for
, endpoints not included.
The second series is
, which we know converges for
.
Putting these intervals together, we see that
converges only on the <em>open</em> interval
.