# Power series whose partial sums attain only finitely many values

I am wondering if there is a general explanation for the following phenomenon. The partial sums of the geometric series $\sum_{n\geq 0} x^n$ evaluated at a root of unity $\zeta\neq 1$ in the complex plane attain only finitely many values (since $1+\zeta+\ldots+\zeta^{n-1}=0$, where $\zeta^n=1$). The average of these values is $1/(1-\zeta)$, which is the value at $\zeta$ of the meromorphic function that analytically continues the geometric series. Out of curiosity, I would like to ask if this is a special case of a general theorem?

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Hopefully this link works: en.wikipedia.org/wiki/Ces%C3%A0ro_mean If not, google "cesaro mean" –  David Cohen May 16 '13 at 1:27
thanks. you link gives only the definition of cesaro mean, however. –  Tommaso Centeleghe May 16 '13 at 12:23

## 1 Answer

The phenomenon you observe is a special case of a theorem of Frobenius (1880):

If a series is Cesaro summable then it is also Abel summable, and the Cesaro limit is the same as the Abel limit.

In your case the series is $(1,\zeta,\zeta^2,\dots)$ which ensures Cesaro summability as the sequence of partial sums is periodic.

For more details see Page 4 of Korevaar: Tauberian theory - A century of developments (Springer, 2004).

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Thanks, this explains exactly what I was asking! –  Tommaso Centeleghe May 16 '13 at 12:22
I am glad I could help! –  GH from MO May 16 '13 at 16:59