Take a natural number's prime factors and list them increasingly and repeating them according to multiplicity. Concatenate their decimal (or in any base) representation to get a new number and repeat the process. Does this always end in a prime number for any input?
[Math] Does listing the prime factors always stop
nt.number-theoryopen-problemsprime numbers
Best Answer
It's open problem, sequence A037274 from OEIS, so-called "home primes". Hm, the value for n=77 is even unknown.
P.S. On-Line Encyclopedia of Integer Sequences definitely should be included in FAQ.