Complex Power Series/Examples

Examples of Complex Power Series

Example: $\displaystyle \sum_{n \mathop \ge 0} n z^n$

$S = \displaystyle \sum_{n \mathop \ge 0} n z^n$

has a radius of convergence of $1$.

Example: $\displaystyle \sum_{n \mathop \ge 0} \dfrac {3^n - 1} {2^n + 1} z^n$

Let $\sequence {a_n}$ be the sequence defined as:

$a_n = \dfrac {3^n - 1} {2^n + 1}$
$S = \displaystyle \sum_{n \mathop \ge 0} a_n z^n$

has a radius of convergence of $\dfrac 2 3$.

Example: $\displaystyle \sum_{n \mathop \ge 0} \dfrac {\paren {2 n}!} {\paren {n!}^2} z^n$

Let $\sequence {a_n}$ be the sequence defined as:

$a_n = \dfrac {\paren {2 n}!} {\paren {n!}^2} z^n$
$S = \displaystyle \sum_{n \mathop \ge 0} a_n z^n$

has a radius of convergence of $\dfrac 1 4$.

Example: $\displaystyle \sum_{n \mathop \ge 0} \dfrac {\cos i n} {n^2} z^n$

Let $\sequence {a_n}$ be the sequence defined as:

$a_n = \dfrac {\cos i n} {n^2} z^n$
$S = \displaystyle \sum_{n \mathop \ge 0} a_n z^n$

has a radius of convergence of $\dfrac 1 e$.