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We say that $A\subset \mathbb{N}$ is an essential component if $d_s(A+B)>d_s(B)$ for every $B\subset \mathbb{N}$ with $0<d_s(B)<1$ where $d_s$ is the Schnirelmann density.


Can a lacunary set $A\subset\mathbb{N}$ be an essential component?
The answer is no by Ruzsa [Ru87], who proved that if $A$ is an essential component then there exists some constant $c>0$ such that $\lvert A\cap \{1,\ldots,N\}\rvert \geq (\log N)^{1+c}$ for all large $N$.

Furthermore, Ruzsa proves that this is best possible, in that for any $c>0$ there exists an essential component $A$ for which $\lvert A\cap \{1,\ldots,N\}\rvert \leq (\log N)^{1+c}$ for all large $N$.

In [Ru99] Ruzsa states "The simplest set with a chance to be an essential component is the collection of numbers in the form $2^m3^n$ and Erdős often asked whether it is an essential component or not; I do not even have a plausible guess."

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This page was last edited 05 October 2025.

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Additional thanks to: Wouter van Doorn

When referring to this problem, please use the original sources of Erdős. If you wish to acknowledge this website, the recommended citation format is:

T. F. Bloom, Erdős Problem #37, https://www.erdosproblems.com/37, accessed 2026-01-16