Prime chains (johndcook.com)

Interesting!

Does anyone know other innovative PoW chains?

This brings me back! During the second year of my bachelor's we had a group project on distributed systems, we basically had to create a blockchain with primecoin PoW, written in service oriented programming language developed at the university of Bologna called Jolie (https://www.jolie-lang.org/)

If you wonder about its financial value, its market cap is around $ 2.5m [1].

[1] https://coinmarketcap.com/currencies/primecoin/

Fun! If bi-twin chains are those where +1 AND -1 are prime, I wonder what the longest chain is where either +1 OR -1 are prime. And while we're at it, why not XOR too.