Past exam of the mathematics course of the University of Cambridge / 2018 / ii / Paper 3 / 12G / a / Solution

The pumping lemma for regular languages states that for every regular language $L$ there is an integer $N$ such that every $w\in L$ with $|w|\ge N$ can be writtenwith

To prove it, let a deterministic finite automaton with $N$ states recognize $L$. During the reading of the first $N$ symbols of an accepted word $w$, the automaton visits $N+1$ states, including the initial state. By the pigeonhole principle, two of them agree, say $q_i=q_j$ with $0\le i<j\le N$. Let $x$ be the first $i$ symbols, $y$ the next $j-i$ symbols, and $z$ the remaining suffix. Reading $y$ is a nonempty closed path from $q_i$ to itself. Traversing this path any number $r\ge 0$ of times and then reading $z$ therefore reaches the same accept state. This proves all three assertions.