# Leonardo number

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

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

Edsger W. Dijkstra[1] used them as an integral part of his smoothsort algorithm,[2] and also analyzed them in some detail.[3]

Computing a second-order recurrence relation recursively and without memoization requires L(n) computations for the n-th item of the series.

## Relation to Fibonacci numbers

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

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:

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

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 $x^2 - x - 1 = 0$.

The first few Leonardo numbers are

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

## References

1. ^ EWD797
2. ^ Dijkstra, Edsger W. Smoothsort – an alternative to sorting in situ (EWD-796a). E.W. Dijkstra Archive. Center for American History, University of Texas at Austin. (original; transcription)
3. ^ EWD796a