Summation over k to n of Product of kth with n-kth Fibonacci Numbers
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Theorem
- $\ds \sum_{k \mathop = 0}^n F_k F_{n - k} = \dfrac {\paren {n - 1} F_n + 2n F_{n - 1} } 5$
where $F_n$ denotes the $n$th Fibonacci number.
Proof
From Generating Function for Fibonacci Numbers, a generating function for the Fibonacci numbers is:
- $\map G z = \dfrac z {1 - z - z^2}$
Then:
\(\ds \map G z\) | \(=\) | \(\ds \dfrac z {1 - z - z^2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 {\sqrt 5} \paren {\dfrac 1 {1 - \phi z} - \dfrac 1 {1 - \hat \phi z} }\) | Partial Fraction Expansion |
where:
- $\phi = \dfrac {1 + \sqrt 5} 2$
- $\hat \phi = \dfrac {1 - \sqrt 5} 2$
Hence:
\(\ds \paren {\map G z}^2\) | \(=\) | \(\ds \paren {\dfrac 1 {\sqrt 5} \paren {\dfrac 1 {1 - \phi z} - \dfrac 1 {1 - \hat \phi z} } }^2\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {\dfrac 1 {1 - \phi z} }^2 + \paren {\dfrac 1 {1 - \hat \phi z} }^2 - 2 \dfrac 1 {1 - \phi z} \dfrac 1 {1 - \hat \phi z} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\dfrac 1 {\paren {1 - \phi z}^2} + \dfrac 1 {\paren {1 - \hat \phi z}^2} - 2 \dfrac {1 - \hat \phi z + 1 - \phi z} {\paren {1 - \phi z} \paren {1 - \hat \phi z} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\dfrac 1 {\paren {1 - \phi z}^2} + \dfrac 1 {\paren {1 - \hat \phi z}^2} - \dfrac 2 {\paren {1 - \phi z} \paren {1 - \paren {1 - \phi} z} } }\) | Definition of $\hat \phi$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\dfrac 1 {\paren {1 - \phi z}^2} + \dfrac 1 {\paren {1 - \hat \phi z}^2} - \dfrac 2 {1 - z - z^2} }\) | after algebra | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\sum_{n \mathop = 0}^\infty \paren {n + 1} \paren {\phi z}^n + \sum_{n \mathop = 0}^\infty \paren {n + 1} \paren {\hat \phi z}^n - \dfrac 2 z \map G z}\) | Power Series Expansion for $\dfrac 1 {\paren {1 - z}^2}$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\sum_{n \mathop = 0}^\infty \paren {n + 1} \paren {\phi^n + \hat \phi^n} z^n - \dfrac 2 z \sum_{n \mathop = 0}^\infty F_n z^n}\) | Generating Function for Fibonacci Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\sum_{n \mathop = 0}^\infty \paren {n + 1} \paren {\phi^n + \hat \phi^n} z^n - 2 \sum_{n \mathop = 0}^\infty F_n z^{n - 1} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\sum_{n \mathop = 0}^\infty \paren {n + 1} \paren {\phi^n + \hat \phi^n} z^n - 2 \sum_{n \mathop = 1}^\infty F_{n + 1} z^n}\) | Translation of Index Variable of Summation | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \sum_{n \mathop = 0}^\infty \paren {\paren {n + 1} \paren {\phi^n + \hat \phi^n} - 2 F_{n + 1} } z^n\) |
Since the coefficient of $z^n$ in $\paren {\map G z}^2$ is $\ds \sum_{k \mathop = 0}^n F_k F_{n - k}$, by comparing coefficients:
\(\ds \sum_{k \mathop = 0}^n F_k F_{n - k}\) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {n + 1} \paren {\phi^n + \hat \phi^n} - 2 F_{n + 1} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {n + 1} \paren {\dfrac {\phi^{2 n} - \hat \phi^{2 n} } {\sqrt 5} \times \dfrac {\sqrt 5} {\phi^n - \hat \phi^n} } - 2 F_{n + 1} }\) | Difference of Two Squares | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {n + 1} \dfrac {F_{2 n} } {F_n} - 2 F_{n + 1} }\) | Euler-Binet Formula | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {n + 1} \dfrac {F_{n - 1} F_n + F_n F_{n + 1} } {F_n} - 2 F_{n + 1} }\) | Honsberger's Identity | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {n + 1} \paren {F_{n - 1} + F_{n + 1} } - 2 F_{n + 1} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 5 \paren {\paren {n + 1} \paren {F_n + 2 F_{n - 1} } - 2 \paren {F_n + F_{n - 1} } }\) | Definition of Fibonacci Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {\paren {n - 1} F_n + 2n F_{n - 1} } 5\) |
$\blacksquare$
Sources
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.8$: Fibonacci Numbers: $(17)$