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Is there a solution/approximation for the non-linear difference equation $c_n = c_{n-1}+c_{\lceil \alpha n \rceil}$, where $0 < \alpha < 1$?

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2 Answers 2

up vote 5 down vote accepted

As Aaron Meyerowitz mentions when $\alpha=\frac{1}{b}$ the sequence is related to the number of partitions of $bn$ into powers of $b$. The asymptotic value of this sequence was determined by de Bruijn (On Mahler's partition problem). I believe his methods can be used to get asymptotic values for general $\alpha$, though I haven't checked this carefully. By looking at the generating function you can reduce it to studying the asymptotics of the coefficients of a power series $F(x)$ (which should be approximately the generating function of your sequence) which satisfies $$F(x)=\frac{1-x^{\lceil 1/\alpha\rceil}}{(1-x)^2}F(x^{\lceil 1/\alpha\rceil})$$ this is a Mahler type functional equation, and there are standard methods in the literature to obtain asymptotics.

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Here is the case $\alpha=1/2.$ with rounding down. It shows conjectures but few results. But you asked about rounding up. That is here and it seems that more is known in this $\alpha=1/2$ case including a good estimte of the growth rate. The $nth$ term is the number of partitions of $2n$ into powers of 2.

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