Fibonacci sequence: Difference between revisions

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=== Limit of consecutive quotients ===

[[Johannes Kepler]] observed that the ratio of consecutive Fibonacci numbers [[convergent sequence|converges]]. He wrote that "as 5 is to 8 so is 8 to 13, practically, and as 8 is to 13, so is 13 to 21 almost", and concluded that these ratios approach the golden ratio <math>{{tmath|\varphi\colon </math> }}:<ref>{{Citation|last=Kepler |first=Johannes |title=A New Year Gift: On Hexagonal Snow |year=1966 |isbn=978-0-19-858120-8 |publisher=Oxford University Press |page= 92}}</ref><ref>{{Citation | title = Strena seu de Nive Sexangula | year = 1611}}</ref>

<math display=block>\lim_{n\to\infty}\frac{F_{n+1}}{F_n}=\varphi.</math>

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Binet's formula provides a proof that a positive integer {{mvar|x}} is a Fibonacci number [[if and only if]] at least one of <math>5x^2+4</math> or <math>5x^2-4</math> is a [[Square number|perfect square]].<ref>{{Citation | title = Fibonacci is a Square | last1 = Gessel | first1 = Ira | journal = [[The Fibonacci Quarterly]] | volume = 10 | issue = 4 | pages = 417–19 |date=October 1972 | url = https://www.fq.math.ca/Scanned/10-4/advanced10-4.pdf | access-date = 2012-04-11 }}</ref> This is because Binet's formula, which can be written as <math>F_n = (\varphi^n - (-1)^n \varphi^{-n}) / \sqrt{5}</math>, can be multiplied by <math>\sqrt{5} \varphi^n</math> and solved as a [[quadratic equation]] in <math>\varphi^n</math> via the [[quadratic formula]]:

<math display=block>\varphi^n = \frac{F_n\sqrt{5} \pm \sqrt{5{F_n}^{\!2} + 4(-1)^n}}{2}.</math>

Comparing this to <math>\varphi^n = F_n \varphi + F_{n-1} = (F_n\sqrt{5} + F_n + 2 F_{n-1})/2</math>, it follows that

: <math display=block>5{F_n}^{\!2} + 4(-1)^n = (F_n + 2F_{n-1})^2\,.</math>

In particular, the left-hand side is a perfect square.

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[[File:Fibonacci Squares.svg|frameless|260x260px]]

=== Symbolic method ===

The sequence <math>(F_n)_{n\in\mathbb N}</math> is also considered using the [[symbolic method (combinatorics)|symbolic method]].<ref>{{citation |last1=Flajolet |first1=Philippe |last2=Sedgewick |first2=Robert |title=Analytic Combinatorics|title-link= Analytic Combinatorics |date=2009 |publisher=Cambridge University Press |isbn=978-0521898065 |page=42}}</ref> More precisely, this sequence corresponds to a [[specifiable combinatorial class]]. The specification of this sequence is <math>\operatorname{Seq}(\mathcal{Z+Z^2})</math>. Indeed, as stated above, the <math>n</math>-th Fibonacci number equals the number of [[Composition (combinatorics)|combinatorial compositions]] (ordered [[integer partition|partitions]]) of <math>n-1</math> using terms 1 and 2.

It follows that the [[ordinary generating function]] of the Fibonacci sequence, <math>\sum_{i=0}^\infty F_iz^i</math>, is the [[rational function]] <math>\frac{z}{1-z-z^2}.</math>

=== Induction proofs ===

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<math display=block>

s(z) = \sum_{k=0}^\infty F_k z^k = 0 + z + z^2 + 2z^3 + 3z^4 + 5z^5 + \dotscdots.

</math>

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Some specific examples that are close, in some sense, to the Fibonacci sequence include:

* Generalizing the index to negative integers to produce the [[negafibonacci]] numbers.

* The [[codenominator function]] provides an extension to positive rationals as an integral-valued map.

* Generalizing the index to [[real number]]s using a modification of Binet's formula.<ref name="MathWorld" />

* Starting with other integers. [[Lucas number]]s have {{math|1=''L''₁ = 1}}, {{math|1=''L''₂ = 3}}, and {{math|1=''L_n'' = ''L''_''n''−1 + ''L''_''n''−2}}. [[Primefree sequence]]s use the Fibonacci recursion with other starting points to generate sequences in which all numbers are composite.

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* [[Mario Merz]] included the Fibonacci sequence in some of his artworks beginning in 1970.{{sfn|Livio|2003|p=176}}

* [[Joseph Schillinger]] (1895–1943) developed [[Schillinger System|a system of composition]] which uses Fibonacci intervals in some of its melodies; he viewed these as the musical counterpart to the elaborate harmony evident within nature.{{sfn|Livio|2003|p=193}} See also {{slink|Golden ratio|Music}}.

* In [[software development]], Fibonacci numbers are often used by [[Agile management|agile]] teams operating under the [[Scrum (software development)|Scrum]] framework to size their [[product backlog]] items.<ref>{{cite web |last1=Kathuria |first1=Madhur |title=A Guide to Using the Fibonacci Sequence in Scrum |url=https://resources.scrumalliance.org/Article/guide-using-fibonacci-sequence-scrum |website=Scrum Alliance |access-date=8 August 2025}}</ref>

== See also ==