Past exam of the mathematics course of the University of Cambridge / 2016 / iii / Paper 120 / 1 / a / ii / Solution

Apply the same pumping of a group multiplier automaton to the accepted pair containing the given representative $u$. Since $|u|>|w|+N$, there is a nonempty loop entirely after the tape containing $w$ has ended. Write $u=pqr$, with $q\ne \epsilon$ the letters read during this loop. Repeating the loop any number of times leaves the other tape equal to $w$, and the group multiplier automaton accepts the resulting padded convolution of words. ConsequentlyThese words over an alphabet have distinct lengths because $|q|>0$. Hence there are infinitely many representatives of $g$ in $L$. No uniqueness of representatives was assumed in either part.