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Define to be maximal such that there exists some choice of congruence class for all primes such that every integer in satisfies at least one of the congruences .
Estimate - in particular is it true that ?
Source: erdosproblems.com/688
No claim settles this problem.
Open. The site labels the problem OPEN. Two results are in hand:
Erdős's lower bound , asserted in
[Er79d] p. 79 ("I can prove") and [Er80] p. 106 ("It is not difficult to prove")
without a proof in either source, which the formal-conjectures collection marks
research solved as a variant, and the elementary counting bound
proved under What is known. No upper bound tending to
, and nothing toward , was found in the search whose
scope the Current assessment records. Erdős's own remark in [Er80] ties the
question to the Erdős--Ruzsa covering conjecture of Problem 1200: if that
conjecture holds "then very likely for some absolute constant
", that is, the answer to the displayed question would be no. This is a
bounded negative finding, not a certificate of openness.