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Fix some integer and define a decreasing sequence in by and, for , letting be the greatest integer in such that all of the prime factors of are .
Is it true that, for sufficiently large , not all of this sequence can be prime?
Fix some integer and define a decreasing sequence in by and, for , letting be the greatest integer in such that all of the prime factors of are .
Is it true that, for sufficiently large , not all of this sequence can be prime?
Source: erdosproblems.com/430
No claim settles this problem.
Open, the site's label (OPEN), which describes the corrected Statement.
The site's wording holds trivially for every . The integer has no prime factors, so it meets the rule vacuously and every sequence ends at it: for the rule gives , and for it gives . Since is not prime, no sequence is all prime. The failure is this page's own elementary check. The change replaces the range by , so that the sequence stops when no integer greater than qualifies; nothing else changes. The evidence is the site's own commentary: its worked example for stops after and , which holds only when is excluded, and it keeps the label OPEN and the equivalence with Problem 385 credited to Sarosh Adenwalla, both of which fit only the corrected question. The defect is already in the poser's text: Erdős and Graham [ErGr80, p. 85] index the sequence from the other end, with and the least integer exceeding for which all prime factors of are greater than , and the value qualifies vacuously in the same way. Their report that Selfridge's preliminary calculations point to a yes answer "but no proof is in sight" fits only the corrected question. Above the excluded value no vacuous case remains, since every integer at least has a prime factor, and the corrected question is a real one: for the sequence is all prime. No result about the site's wording exists beyond the trivial check recorded here.