Generating Function for Constant Sequence
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Theorem
Let $\sequence {a_n}$ be the sequence defined as:
- $\forall n \in \N: a_n = r$
for some $r \in \R$.
Then the generating function for $\sequence {a_n}$ is given as:
- $\map G z = \dfrac r {1 - z}$ for $\size z < 1$
Proof
\(\ds \map G z\) | \(=\) | \(\ds \sum_{n \mathop = 0}^\infty r z^n\) | Definition of Generating Function | |||||||||||
\(\ds \) | \(=\) | \(\ds r \sum_{n \mathop = 0}^\infty z^n\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac r {1 - z}\) | Sum of Infinite Geometric Sequence |
for $\size z < 1$.
$\blacksquare$
Examples
$a_0 = 1, a_n = 2$
Let $\sequence {a_n}$ be the sequence defined as:
- $\forall n \in \Z_{\ge 0}: a_n = \begin{cases} 1 & : n = 0 \\ 2 & : n > 0 \end{cases}$
Then the generating function for $\sequence {a_n}$ is given as:
- $\map G z = \dfrac {1 + z} {1 - z}$
Sources
- 1971: George E. Andrews: Number Theory ... (previous) ... (next): $\text {3-4}$ Generating Functions: Example $\text {3-6}$
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.9$: Generating Functions: $(5)$