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Intertwined infinite binary words. (English) Zbl 0856.68117
Summary: In [\((*)\) “Gödel, Escher, Bach. An Eternal Golden Braid” (New York, 1979)] D. Hofstadter defines the sequence \(h(n)\) by \[ h(n)= n- h(h(n- 1)),\quad n> 1,\tag{1} \] with \(h(1)= 1\) and remarks on its close relation to the sequence \(F_n\) of Fibonacci numbers given by the well-known formula \[ F_n= F_{n- 1}+ F_{n- 2},\quad n> 2,\tag{2} \] with \(F_1= F_2= 1\). As it was proved by P. J. Downey and R. E. Griswold [“On a family of nested recurrences”, Fibonacci Q. 22, 310-317 (1984; Zbl 0547.10009)], \[ h(n)= F_{r- 1}+ F_{s- 1}+\cdots+ F_{t- 1},\quad n> 1,\tag{3} \] if \(n= F_r+ F_s+\cdots+ F_t\) is the Zeckendorf expansion of \(n\). Moreover, an explicit formula \[ h(n)= [\mu(n+ 1)],\quad n> 0,\tag{4} \] was found there, where \([.]\) denotes the greatest integer function and \(\mu= (- 1+ \sqrt 5)/2\) is the golden ratio. Similar but less known “married” sequences \(f(n)\) and \(g(n)\) defined by \[ f(n)= n- g(f(n- 1)),\quad g(n)= n- f(g(n- 1)),\quad n> 0,\tag{5} \] with \(f(0)= 1\), \(g(0)= 0\) are introduced also in \((*)\).
In this note, we generalize sequences (1) and (5) and derive an asymptotic result for new sequences. However, we shall come to our generalization from quite a different side, namely, from cryptography and symbolic dynamics.

MSC:
68R15 Combinatorics on words
11B83 Special sequences and polynomials
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