Unimodality of the distribution of the number of prime divisors of an integer. (Unimodalité de la distribution du nombre de diviseurs premiers d’un entier.) (French) Zbl 0711.11030

Let \(\Omega(n)\) (resp. \(\omega(n)\)) denote the number of prime factors of \(n\), counted with (resp. without) multiplicity, and set \[ \pi(x,k)=\#\{n\leq x: \omega(n)=k\},\quad \sigma(x,k)=\#\{n\leq x: \Omega(n)=k\}, \]
\[ \rho(x,k)=\#\{n\leq x+\mu^2(n)=1,\ \omega(n)=k\}. \]
It had been conjectured by Erdős that each of these functions is unimodal in \(k\) for sufficiently large \(x\), i.e., there exists an integer \(k_0=k_0(x)\) such that the function is non-decreasing for \(k\leq k_0\) and non-increasing for \(k\geq k_0\). The author recently [Sémin. Théor. Nombres, Paris 1987–88, Prog. Math. 81, 1–21 (1990; Zbl 0689.10047)] established Erdős’ conjecture for the function \(\sigma(x,k)\). Now he proves the conjecture for \(\pi(x,k)\) and remarks that a similar argument yields the conjecture for \(\rho(x,k)\).
The argument consists of two parts. Using a more precise version of the Sathe-Selberg formula for \(\pi(x,k)\) [A. Selberg, J. Indian Math. Soc., New Ser. 18, 83–87 (1954; Zbl 0057.28502)] he shows first that, for any fixed constant \(C\) and all sufficiently large \(x\), one has \(\pi(x,k+1)\geq \pi(x,k)\) for \(k<k_0\) and \(\pi(x,k+1)\leq \pi(x,k)\) for \(k_0\leq k\leq C\log\log x\) with a suitable integer \(k_0=k_0(x)\) satisfying \(k_0=\log \log x+O(1).\)
He then proves that the second inequality persists for \(k\geq C \log \log x\) by showing more generally that for any fixed constant \(\mu>1\), all sufficiently large \(x\) and \(3/2\leq t\leq x\), the function \(\pi(x,t,k)=\#\{n\leq x: p\mid n\Rightarrow p>t, \omega(n)=k\}\) is decreasing in \(k\geq \mu \log (\log x/\log t).\) To establish the latter result, the author uses the functional equation \(\pi(x,t,k+1)= \sum_{p>t, m\geq 1}\pi(xp^{-m},p,k)\) and an asymptotic estimate for \(\pi(x,t,k)\) due to K. Alladi [Q. J. Math., Oxf. II. Ser. 33, 129–148 (1982; Zbl 0483.10049)].


11N25 Distribution of integers with specified multiplicative constraints
11N37 Asymptotic results on arithmetic functions
Full Text: DOI Numdam EuDML


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