© 2008 London Mathematical Society
Multiplicative decomposability of shifted sets
Department of Mathematics
Royal Holloway
Egham
Surrey TW20 0EX
United Kingdom
Received 25 May 2007. Revision received 27 July 2007.
The following two problems are open.
- Do two sets of positive integers
and
exist, with at least two elements each, such that
+
coincides with the set of primes
for sufficiently large elements?
- Let
={6, 12, 18}. Is there an infinite set
of positive integers such that 
+1
? A positive answer would imply that there are infinitely many Carmichael numbers with three prime factors.
In this paper we prove the multiplicative analogue of the first problem, namely that there are no two sets
and
, with at least two elements each, such that the product 
coincides with any additively shifted copy
+c of the set of primes for sufficiently large elements. We also prove that shifted copies of sets of integers that are generated by certain subsets of the primes cannot be multiplicatively decomposed.
2000 Mathematics Subject Classification 11N25, 11N36 (primary).