Neither of those papers contains a proof-by-contradiction or a counterexample question, as far as I can tell.
The canonical example of a proof by contradiction is Euclid's proof that there are infinitely many primes, which is in the spoiler (but if you want to practise contradiction, you might like to leave this as an exercise, with the hint that "you can create a number which is not divided by any numbers in a given list, by multiplying the list together and adding 1").
I don't think I've ever seen it on a paper, that I remember (I took A-levels two years ago, so my memory may be faulty). It's quite hard to practise, too.