For an Erd?os-Sur′anyi sequence it is customary to consider its signum equation. Based on some classical heuristic arguments, we conjecture the asymptotic behavior for the number of solutions of this signum equation in the case of the sequence {nk}n (k 2) and the sequence of primes. Surprisingly, we show that this method does not apply at all for the Fibonacci sequence. By computing the precise number of solutions, in this case, we obtain an exponential growth, which shows, in particular, the limitations of such an intuition.
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