The notion of PoW predates hashcash. Hashcash is just the simplest PoW algorithm. There are PoW that work entirely differently from Hashcash, such as finding fi

The notion of PoW predates hashcash. Hashcash is just the simplest PoW algorithm. There are PoW that work entirely differently from Hashcash, such as finding fixed length cycles in random bipartite graphs.