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A prime is in class if the only prime divisors of are or . In general, a prime is in class if every prime factor of is in some class , with equality for at least one prime factor.
Are there infinitely many primes in each class? If is the least prime in class , then how does behave?
A prime is in class if the only prime divisors of are or . In general, a prime is in class if it is in no class and every prime factor of is in some class , with equality for at least one prime factor.
Are there infinitely many primes in each class? If is the least prime in class , then how does behave?
Source: erdosproblems.com/1055
No claim settles this problem.
Open; the site labels the problem OPEN.
As the site words it, the classes are not disjoint and the second question is trivial. The prime is in class , since and is in class (); by induction on , every prime of class is in every class, since and lie in class , is even for odd , and . So the least prime of every class is and ; a computation over the primes below confirms that each of the classes to , so read, has least element and contains class . The corrected Statement inserts "it is in no class and", so that each prime has exactly one class, the least for which the condition holds. The defect is already in Guy's A18 [Gu04, p. 66], "The Erdős–Selfridge classification of primes", which gives the definition in the site's words but states it as Erdős and Selfridge's classification of the primes, tables classes to as disjoint sets (class begins and class begins ), and gives the least primes of classes to as . The site's commentary gives the same sequence of least primes (A005113 in the OEIS). The same computation gives these least primes under the corrected Statement, followed by and for classes and , the first entries of Guy's tables. The site also cites [Er77], which the corpus has not read. The formal-conjectures statement adopts the corrected definition, excluding the lower classes since its revision of 2026-08-16. Class is the same set in both forms, and no result about the site's wording is recorded.