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Systèmes de numération et fonctions fractales relatifs aux substitutions. (Numeration systems and fractal functions related to substitutions). (French) Zbl 0679.10010
Let s(n) be the number of ones in the binary expansion of the integer n, and r(n) be the number of (possibly overlapping) 11’s in this expansion. J. Coquet proved [Invent. Math. 73, 107-115 (1983; Zbl 0528.10006)] that the sum $$\sum_{n<N}(-1)^{s(3n)}$$ has an oscillating behaviour. More precisely $\sum_{n<N}(-1)^{s(3n)}=N^{Log 3/Log 4}F(N)+O(1),$ where F is a real continuous and nowhere differentiable function such that $$F(4x)=F(x)$$. J. Brillhart, P. Erdős and P. Morton [Pac. J. Math. 107, 39-69 (1983; Zbl 0469.10034)] proved a similar result for r(n), indeed $$\sum_{n<N}(-1)^{r(n)}\sim \sqrt{N} G(N)$$ where G is continuous, almost nowhere differentiable, and satisfies $$G(4x)=G(x).$$
In the paper under review the authors considerably generalize these results: for a wide class of sequences v, obtained as images of fixed points of certain substitutions (not necessarily of constant length) one has $\sum_{n<N}v_ n\quad \sim \quad C(Log N)^{\alpha} N^{\beta} F(N),$ where F is continuous and multiplicatively periodic. The main tool is a “numeration system associated to a fixed point of a substitution”.
Note that, showing that $$x^{\beta}F(x)$$ is self-affine (generalization of the definition of T. Kamae [Japan J. Appl. Math. 3, 271-280 (1986; Zbl 0646.28005)], the authors prove that F is nowhere differentiable (which implies that the function G in the second example above is actually nowhere differentiable).
Reviewer: J.-P.Allouche

##### MSC:
 11A63 Radix representation; digital problems 11B99 Sequences and sets 68Q42 Grammars and rewriting systems
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##### References:
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