# Leonardo number

Short description: Set of numbers used in the smoothsort algorithm

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

$\displaystyle{ L(n) = \begin{cases} 1 & \mbox{if } n = 0 \\ 1 & \mbox{if } n = 1 \\ L(n - 1) + L(n - 2) + 1 & \mbox{if } n \gt 1 \\ \end{cases} }$

Edsger W. Dijkstra used them as an integral part of his smoothsort algorithm, and also analyzed them in some detail. 

A Leonardo prime is a Leonardo number that's also prime.

## 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:
$\displaystyle{ L(n)=2L(n-1)-L(n-3) }$

## Relation to Fibonacci numbers

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

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:

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

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