Proving $ww^Ru$ is not a regular language with Pumping Lemma

Question

I'm trying to prove that $L=\{ww^Ru:w,u∈\{a,b\}^+\}$ ($w^R$ is the reverse of $w$)

$w$ and $u$ cannot be empty strings.

I want to prove this by using pumping lemma but I cannot find a good starting string.

Answer 1

I would suggest $ww^R$ with $w=ababbabbbabbbb ...$ i.e. increasing sequences of $b$'s separated by an $a$.

I think it's easy to find a contradiction to the pumping lemma using this string.