### Copyright Notice

The following manuscript

EWD570 An exercise for Dr.R.M.Burstall

is held in copyright by Springer-Verlag New York. The manuscript was published as pages 215–216 of

Edsger W. Dijkstra, Selected Writings on Computing: A Personal Perspective, Springer-Verlag, 1982. ISBN 0–387–90652–5.

Reproduced with permission from Springer-Verlag New York. Any further reproduction is strictly prohibited.

 27th May 1976 EWD 570

An exercise for Dr.R.M.Burstall

Dear Rod,

because —as you know— we Dutch are a God-fearing nation, Ascension-day is here an official Holiday, and on official Holidays I don't work. Today I just fooled with figures.

In doing so I discovered a function of the natural numbers which has a nice recursive definition, viz.

fusc(1) = 1
fusc(2n) = fusc(n)
fusc(2n+1) = fusc(n) + fusc(n+1)

a definition which, as far as complexity is concerned, seems to lie between the Fibonacci series and the Pascal triangle.

(The function fusc is of a mild interest on account of the following property: with  f1 = fusc(n1)  and  f2 = fusc(n2)  the following two statements hold:
"if there exists an  N  such that  n1 + n2 = 2N , then  f1  and  f2  are relatively prime" and "if  f1  and  f2  are relatively prime, then there exists an  n1 , an  n2 , and an  N , such that  n1 + n2 = 2N ." In the above recursive definition, this is no longer obvious, at least not to me; hence its name.)

Having seen your exercises concerning the derivation of an iterative program, starting with the recursive definition for the n-th number of the Fibonacci series, I was suddenly reminded of that exercise when I was considering an iterative program for the computation of  fusc . It should be a rewarding exercise, as there exists a very nice iterative program:

n, a, b := N, 1, 0;
do n ≠ 0 and even(n) → a, n:= a + b, n/2
▯ odd(n) → b, n:= b + a, (n-1)/2
od {b = fusc(N)}

I wish you luck and enjoyment!         Yours ever,

 Plataanstraat 5 NL-4565 NUENEN The Netherlands prof.dr. Edsger W. Dijkstra Burroughs Research Fellow