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Is there possibly a largest prime number?

Prime numbers are numbers with no factors other than one and itself. Factors of a number are always lower or equal to than a given number; so, the larger the number is, the larger the pool of "...
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Prove that the language $\{ww \mid w \in \{a,b\}^*\}$ is not FA (Finite Automata) recognisable.
Hint: Assume that $|xy| \le k$ in the pumping lemma. I have no idea where to begin for this. Any help would be much appreciated.