Post's Correspondence Problem (PCP)

A post correspondence system consists of a finite set of ordered pairs where for some alphabet .

Any sequence of numbers

is called a solution to a Post Correspondence System.

The Post's Correspondence Problem is the problem of determining whether a Post Correspondence system has a solutions.

Example 1 : Consider the post correspondence system

                                The list 1,2,1,3 is a solution to it.

Because