It is known to be undecidable when 9 pairs are used , but Stephen Wolfram suggested that it is also undecidable with just 3 pairs .
With 2 string pairs , PCP was shown to be decidable in 1981 .
PCP is known to be undecidable when 9 pairs are used , but Stephen Wolfram suggested that it is also undecidable with just 3 pairs .
With 2 string pairs , PCP was shown to be decidable in 1981 .