I just learned pumping lemma this week and got confused on this question.
B={$a^{fn}$ | $f_n$ is a Fibonacci number} for $a \in Σ$.
Hint: the sequence of Fibonacci numbers get increasingly further apart.
I am trying to prove this is not regular using pumping lemma, but 'Fibonacci numbers get increasingly further apart' seems really tricky when I try to pump up to prove it's not $\notin$ B.