

It would have to be Gödel’s Incompleteness Theorem. Such a beautiful proof that shakes mathematics to its core.
The science communicator Veritasium made a nice video about it: https://youtu.be/HeQX2HjkcNo
I first learned about it in Douglas Hofstaedter’s masterpiece Gödel Escher Bach: An Eternal Golden Braid
I have had some issues with Robert Oster inks and hard starts. In particular, they don’t work well in my Lamy 2000 fine nib and Pilot Prera medium nib pens, whereas a Pilot Iroshizuku ink I bought recently seems to flow a lot better. It’s a shame, because I do like the colours; I might try them in my Kaweco sport pen (which has a wetter medium nib) and see if they flow better.