Leonardo number

Last updated February 25, 2023

The Leonardo numbers are a sequence of numbers given by the recurrence:

Values

The first Leonardo numbers are

1, 1, 3, 5, 9, 15, 25, 41, 67, 109, 177, 287, 465, 753, 1219, 1973, 3193, 5167, 8361, ... (sequence A001595 in the OEIS )

The only Leonardo primes are

3, 5, 41, 67, 109, 1973, 5167, 2692537, 11405773, 126491971, 331160281, 535828591, 279167724889, 145446920496281, 28944668049352441, 5760134388741632239, 63880869269980199809, 167242286979696845953, 597222253637954133837103, ... (sequence A145912 in the OEIS )

Modulo cycles

The Leonardo numbers form a cycle in any modulo n≥2. An easy way to see it is:

If a pair of numbers modulo n appears twice in the sequence, then there's a cycle.
If we assume the main statement is false, using the previous statement, then it would imply there's infinite distinct pairs of numbers between 0 and n-1, which is false since there are n² such pairs.

The cycles for n≤8 are:

Modulo	Cycle	Length
2	1	1
3	1,1,0,2,0,0,1,2	8
4	1,1,3	3
5	1,1,3,0,4,0,0,1,2,4,2,2,0,3,4,3,3,2,1,4	20
6	1,1,3,5,3,3,1,5	8
7	1,1,3,5,2,1,4,6,4,4,2,0,3,4,1,6	16
8	1,1,3,5,1,7	6

The cycle always end on the pair (1,n-1), as it's the only pair which can precede the pair (1,1).

Expressions

The following equation applies:

L(n)=2L(n-1)-L(n-3)

Proof

$L(n)=L(n-1)+L(n-2)+1=L(n-1)+L(n-2)+1+L(n-3)-L(n-3)=2L(n-1)-L(n-3)$

Relation to Fibonacci numbers

The Leonardo numbers are related to the Fibonacci numbers by the relation $L(n)=2F(n+1)-1,n\geq 0$ .

From this relation it is straightforward to derive a closed-form expression for the Leonardo numbers, analogous to Binet's formula for the Fibonacci numbers:

L(n)=2{\frac {\varphi ^{n+1}-\psi ^{n+1}}{\varphi -\psi }}-1={\frac {2}{\sqrt {5}}}\left(\varphi ^{n+1}-\psi ^{n+1}\right)-1=2F(n+1)-1

where the golden ratio $\varphi =\left(1+{\sqrt {5}}\right)/2$ and $\psi =\left(1-{\sqrt {5}}\right)/2$ are the roots of the quadratic polynomial $x^{2}-x-1=0$ .

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References

↑ "E.W.Dijkstra Archive: Fibonacci numbers and Leonardo numbers. (EWD 797)". www.cs.utexas.edu. Retrieved 2020-08-11.
↑ Dijkstra, Edsger W. Smoothsort – an alternative to sorting in situ (EWD-796a) (PDF). E.W. Dijkstra Archive. Center for American History, University of Texas at Austin. ( transcription )
↑ "E.W.Dijkstra Archive: Smoothsort, an alternative for sorting in situ (EWD 796a)". www.cs.utexas.edu. Retrieved 2020-08-11.
↑ "Leonardo Number - GeeksforGeeks". www.geeksforgeeks.org. Retrieved 2022-10-08.

External links

OEIS sequenceA001595

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[1] "E.W.Dijkstra Archive: Fibonacci numbers and Leonardo numbers. (EWD 797)". www.cs.utexas.edu. Retrieved 2020-08-11.

[2] Dijkstra, Edsger W. Smoothsort – an alternative to sorting in situ (EWD-796a) (PDF). E.W. Dijkstra Archive. Center for American History, University of Texas at Austin. ( transcription )

[3] "E.W.Dijkstra Archive: Smoothsort, an alternative for sorting in situ (EWD 796a)". www.cs.utexas.edu. Retrieved 2020-08-11.

[4] "Leonardo Number - GeeksforGeeks". www.geeksforgeeks.org. Retrieved 2022-10-08.

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