Lucas number explained

The Lucas sequence is an integer sequence named after the mathematician François Édouard Anatole Lucas (1842 - 1891), who studied both that sequence and the closely related Fibonacci sequence. Individual numbers in the Lucas sequence are known as Lucas numbers. Lucas numbers and Fibonacci numbers form complementary instances of Lucas sequences.

The Lucas sequence has the same recursive relationship as the Fibonacci sequence, where each term is the sum of the two previous terms, but with different starting values.^[1] This produces a sequence where the ratios of successive terms approach the golden ratio, and in fact the terms themselves are roundings of integer powers of the golden ratio.^[2] The sequence also has a variety of relationships with the Fibonacci numbers, like the fact that adding any two Fibonacci numbers two terms apart in the Fibonacci sequence results in the Lucas number in between.^[3]

The first few Lucas numbers are

2, 1, 3, 4, 7, 11, 18, 29, 47, 76, 123, 199, 322, 521, 843, 1364, 2207, 3571, 5778, 9349, ... .

C _{n} of length n\geq 2 .

Definition

As with the Fibonacci numbers, each Lucas number is defined to be the sum of its two immediately previous terms, thereby forming a Fibonacci integer sequence. The first two Lucas numbers are

L _{0=2} and L _{1=1} , which differs from the first two Fibonacci numbers F _{0=0} and F _{1=1} . Though closely related in definition, Lucas and Fibonacci numbers exhibit distinct properties.

The Lucas numbers may thus be defined as follows:

L _{n} : = \begin{cases} 2 & if n = 0 ; \\ 1 & if n = 1 ; \\ L _{n-1} + L _{n-2} & if n > 1 . \end{cases} (where n belongs to the natural numbers)

All Fibonacci-like integer sequences appear in shifted form as a row of the Wythoff array; the Fibonacci sequence itself is the first row and the Lucas sequence is the second row. Also like all Fibonacci-like integer sequences, the ratio between two consecutive Lucas numbers converges to the golden ratio.

Extension to negative integers

Using

L _{n-2} = L _{n} - L _{n-1} , one can extend the Lucas numbers to negative integers to obtain a doubly infinite sequence:

..., -11, 7, -4, 3, -1, 2, 1, 3, 4, 7, 11, ... (terms

L _{n} for -5 \leq{}n\leq 5 are shown).The formula for terms with negative indices in this sequence isL _{-n} = ( -1 ) ^{nL} _{n.}

Relationship to Fibonacci numbers

The Lucas numbers are related to the Fibonacci numbers by many identities. Among these are the following:

L _{n} = F _{n-1} + F _{n+1} = 2 F _{n+1} - F _{n} L _{m+n} = L _{m+1} F _{n} + L _{mF} _{n-1} F _{2n} = L _{n} F _{n} F _{n+k} + ( -1 ) ^{k} F _{n-k} = L _{k} F _{n} 2 F _{2n+k} = L _{n} F _{n+k} + L _{n+k} F _{n} L _{2n} = 5 F ^{2} _{n} + 2 ( -1 ) ^{n} = L ^{2} _{n} - 2 ( -1 ) ^{n} , so \lim _{n\toinfty} (L _{n})/(F _{n}) = \sqrt{ 5 } .\vert L _{n} - \sqrt{ 5 } F _{n} \vert = (2)/(\varphi ^{n}) \to 0 L _{n+k} - ( -1 ) ^{k} L _{n-k} = 5 F _{n} F _{k} ; in particular, F _{n} = {L _{n-1} + L _{n+1} \over 5 } , so 5 F _{n} + L _{n} = 2 L _{n+1} .

Their closed formula is given as:

L _{n} = \varphi ^{n} + ( 1- \varphi) ^{n} = \varphi ^{n} + ( - \varphi) ^{-n} = \left({ 1+ \sqrt{ 5 } \over 2 }\right) ^{n} + \left({ 1- \sqrt{ 5 } \over 2 }\right) ^{n} ,

where

\varphi is the golden ratio. Alternatively, as for n> 1 the magnitude of the term ( - \varphi) ^{-n} is less than 1/2, L _{n} is the closest integer to \varphi ^{n} or, equivalently, the integer part of \varphi ^{n+1/2} , also written as \lfloor \varphi ^{n+1/2} \rfloor .

Combining the above with Binet's formula,

F _{n} = (\varphi ^{n} - ( 1- \varphi) ^{n})/(\sqrt{ 5) }\,,

a formula for

\varphi ^{n} is obtained:\varphi ^{n} = {{L _{n} + F _{n} \sqrt{ 5 }} \over 2 } .

For integers n ≥ 2, we also get:

\varphi ^{n} = L _{n} - ( - \varphi) ^{-n} = L _{n} - ( -1 ) ^{n} L ^{-1} _{n} - L ^{-3} _{n} + R

with remainder R satisfying

\vert R \vert < 3 L ^{-5} _{n} .

Lucas identities

Many of the Fibonacci identities have parallels in Lucas numbers. For example, the Cassini identity becomes

L ^{2} _{n} - L _{n-1} L _{n+1} = ( -1 ) ^{n} 5

Also

\sum ^{n} _{k =0} L _{k} = L _{n+2} - 1 \sum ^{n} _{k =0} L ^{2} _{k} = L _{nL} _{n+1} + 2 2 L ^{2} _{n -1} + L ^{2} _{n} = L _{2n+1} + 5 F ^{2} _{n -2}

where

style F _{n = (L _{n-1} + L _{n+1})/(5)} .L ^{k} _{n} = \sum ^{\lfloor (k)/(2) \rfloor} _{j =0} ( -1 ) ^{nj} \binom{k}{j} L' _{(k-2j)n}

where

L' _{n=L} _{n} except for L' _{0=1} .

For example if n is odd,

L ^{3} _{n} = L' _{3n} -3 L' _{n} and L ^{4} _{n} = L' _{4n} -4 L' _{2n} +6 L' _{0}

Checking,

L _{3=4,} 4 ^{3=64=76-3(4)} , and 256=322-4 ( 18 ) +6

Generating function

Let

\Phi(x) = 2 + x + 3 x ^{2} + 4 x ^{3} + … = \sum ^{inf ty} _{n =0} L ^{n} _{nx}

be the generating function of the Lucas numbers. By a direct computation,

\begin{align} \Phi(x) & = L _{0} + L _{1x} + \sum ^{inf ty} _{n =2} L ^{n} _{nx} \\ & = 2 + x + \sum ^{inf ty} _{n =2} (L _{n} + L _{n} )x ^{n} \\ & = 2 + x + \sum ^{inf ty} _{n =1} L ^{n +1} _{nx} + \sum ^{inf ty} _{n =0} L ^{n +2} _{nx} \\ & = 2 + x + x(\Phi(x) - 2 ) + x ^{2} \Phi(x) \end{align}

which can be rearranged as

\Phi(x) = (2- x)/(1- x - x ^{2}) \Phi\left( - (1)/(x) \right) gives the generating function for the negative indexed Lucas numbers, \sum ^{inf ty} _{n =0} ( -1 ) ^{- n} _{nx} = \sum ^{inf ty} _{n =0} L _{-n} x ^{-n} , and\Phi\left( - (1)/(x) \right) = (x +2 x ^{2})/(1- x - x ^{2}) \Phi(x) satisfies the functional equation\Phi(x) - \Phi\left( - (1)/(x) \right) = 2

As the generating function for the Fibonacci numbers is given by

s(x) = (x)/(1- x - x ^{2})

we have

s(x) + \Phi(x) = (2)/(1- x - x ^{2})

which proves that

F _{n} + L _{n} = 2 F _{n+1} ,

and

5 s(x) + \Phi(x) = (2 x\Phi( - (1 x))/(=))/(2 (1)/(1- x - x ^{2})) +4 (x)/(1- x - x ^{2})

proves that

5 F _{n} + L _{n} = 2 L _{n+1}

The partial fraction decomposition is given by

\Phi(x) = (1)/(1- \phi x) + (1)/(1- \psi x)

where

\phi = (1+ \sqrt{ 5)/() } is the golden ratio and \psi = (1- \sqrt{ 5)/() } is its conjugate.

This can be used to prove the generating function, as

\sum ^{inf ty} _{n =0} L ^{n} _{nx} = \sum ^{inf ty} _{n =0} (\phi ^{n} + \psi ^{n)x} ^{n} = \sum ^{inf ty} _{n =0} \phi ^{nx} ^{n} + \sum ^{inf ty} _{n =0} \psi ^{nx} ^{n} = (1)/(1- \phi x) + (1)/(1- \psi x) = \Phi(x)

Congruence relations

If

F _{n\geq} 5 is a Fibonacci number then no Lucas number is divisible by F _{n} .L _{n} is congruent to 1 modulo n if n is prime, but some composite values of n also have this property. These are the Fibonacci pseudoprimes.L _{n-L} _{n-4} is congruent to 0 modulo 5.

Lucas primes

A Lucas prime is a Lucas number that is prime. The first few Lucas primes are

2, 3, 7, 11, 29, 47, 199, 521, 2207, 3571, 9349, 3010349, 54018521, 370248451, 6643838879, ... .

The indices of these primes are (for example, L₄ = 7)

0, 2, 4, 5, 7, 8, 11, 13, 16, 17, 19, 31, 37, 41, 47, 53, 61, 71, 79, 113, 313, 353, 503, 613, 617, 863, 1097, 1361, 4787, 4793, 5851, 7741, 8467, ... ., the largest confirmed Lucas prime is L₁₄₈₀₉₁, which has 30950 decimal digits.^[4], the largest known Lucas probable prime is L_5466311, with 1,142,392 decimal digits.^[5]

If L_n is prime then n is 0, prime, or a power of 2.^[6] L_2^m is prime for m = 1, 2, 3, and 4 and no other known values of m.

Lucas polynomials

L _{n} (x) are a polynomial sequence derived from the Lucas numbers.

Continued fractions for powers of the golden ratio

Close rational approximations for powers of the golden ratio can be obtained from their continued fractions.

For positive integers n, the continued fractions are:

\varphi ^{2n-1} = [L _{2n-1} ; L _{2n-1} , L _{2n-1} , L _{2n-1} , \ldots] \varphi ^{2n} = [L _{2n} -1 ; 1 , L _{2n} -2 , 1 , L _{2n} -2 , 1 , L _{2n} -2 , 1 , \ldots] .

For example:

\varphi ^{5} = [ 11 ; 11 , 11 , 11 , \ldots] is the limit of(11)/(1) , (122)/(11) , (1353)/(122) , (15005)/(1353) , \ldots with the error in each term being about 1% of the error in the previous term; and\varphi ^{6} = [ 18 - 1 ; 1 , 18 - 2 , 1 , 18 - 2 , 1 , 18 - 2 , 1 , \ldots] = [ 17 ; 1 , 16 , 1 , 16 , 1 , 16 , 1 , \ldots] is the limit of(17)/(1) , (18)/(1) , (305)/(17) , (323)/(18) , (5473)/(305) , (5796)/(323) , (98209)/(5473) , (104005)/(5796) , \ldots with the error in each term being about 0.3% that of the second previous term.

Applications

Lucas numbers are the second most common pattern in sunflowers after Fibonacci numbers, when clockwise and counter-clockwise spirals are counted, according to an analysis of 657 sunflowers in 2016.^[7]

See also

• Generalizations of Fibonacci numbers

External links

• • • • "The Lucas Numbers", Dr Ron Knott
• Lucas numbers and the Golden Section
• A Lucas Number Calculator can be found here.

Notes and References

1. Web site: Weisstein. Eric W.. Lucas Number. 2020-08-11. mathworld.wolfram.com. en.
2. Book: Parker . Matt . Things to Make and Do in the Fourth Dimension . 2014 . Farrar, Straus and Giroux . 978-0-374-53563-6 . 284 . English . 13.
3. Book: Parker . Matt . Things to Make and Do in the Fourth Dimension . 2014 . Farrar, Straus and Giroux . 978-0-374-53563-6 . 282 . English . 13.
4. Web site: The Top Twenty: Lucas Number . primes.utm.edu . 6 January 2022.
5. Web site: Henri & Renaud Lifchitz's PRP Top - Search by form . www.primenumbers.net . 6 January 2022.
6. Chris Caldwell, "The Prime Glossary: Lucas prime" from The Prime Pages.
7. Swinton . Jonathan . Ochu . Erinma . null . null . Novel Fibonacci and non-Fibonacci structure in the sunflower: results of a citizen science experiment . Royal Society Open Science . 2016 . 3 . 5 . 160091 . 10.1098/rsos.160091 . 4892450 . 27293788. 2016RSOS....360091S .