Best guess for the function is
By the ratio test, the series converges for
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
which also converges for by the ratio test:
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
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 .