There is a story in a similar spirit about showing that one of the supposed Mersenne primes (2^67-1) was in fact composite -- (probably apocryphally) by simply

There is a story in a similar spirit about showing that one of the supposed Mersenne primes (2^67-1) was in fact composite -- (probably apocryphally) by simply doing the arithmetic by hand on a blackboard during a talk. For more details, see: - https://en.wikipedia.org/wiki/Mersenne_prime https://en.wikipedia.org/wiki/Mersenne_prime - https://hsm.stackexchange.com/questions/2105/whats-the-famous-story-about-a-mathematician-who-gave-a-talk-without-saying-a-w/2106 https://hsm.stackexchange.com/questions/2105/whats-the-famou... - https://mathoverflow.net/questions/207321/how-did-cole-factor-267-1-in-1903 https://mathoverflow.net/questions/207321/how-did-cole-facto...