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Let $S$ be the set of integers which are a product of $k$ distinct primes, $k$ a fixed positive integer (the condition that the primes are distinct is not crucial). Landau used the Prime Number Theorem to prove that $$\frac{1}{x}\sum_{n \in S:\, n \le x} = \frac{\log^{k-1} \log x}{(k-1)!\log x} (1+o(1)),$$ and he obtains some information on the size of the error term.

I am trying to find what is known, conditionally (on RH) and unconditionally, on the asymptotics of $\sum_{n\in S: x\le n \le x+x^{c}} 1$ as $x$ goes to infinity. Landau's result answers this for $c=1$.

For $k=1$, I am just asking about actual primes in short intervals. RH gives the asymptotics when $c>1/2$, and the work of Huxley gives the asymptotics unconditionally when $c>7/12$. For $k>1$ all I can find are 'almost everywhere' results. What results are known for all intervals when $k>1$?

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    $\begingroup$ Why post a question and then answer it yourself one hour later? $\endgroup$ Sep 27 '18 at 17:47
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    $\begingroup$ @GregMartin I did not find anything relevant for quite a while before posting the question here -- I really thought that perhaps an expert is required here. In the meanwhile I continued sifting through the literature and actually found the right paper -- not sure if by luck or `skill'. $\endgroup$ Sep 27 '18 at 17:53
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Kátai, building on an important paper of Ramachandra, was able to prove that $$\sum_{n \in S: x \le n \le x+x^c} 1 \sim x^c \frac{\sum_{n \in S: n \le x}}{x}$$ as $x \to \infty$, for $c=\frac{1}{2}+\varepsilon$ conditionally and $c=\frac{7}{12}+\varepsilon$ unconditionally. Moreover, he achieves some uniformity in the parameter $k$.

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    $\begingroup$ Nice. Could you please specify Landau's result in either the question or the answer to make the answer more explicit. $\endgroup$
    – kodlu
    Sep 27 '18 at 9:07
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    $\begingroup$ @kodlu Thanks for the suggestion, I've edited the question accordingly. $\endgroup$ Sep 27 '18 at 9:46

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