> A good proof of work is difficult to calculate and easy to verify (i.e. an extremely low hash value). That's a specific Proof-of-Work system known as Hashcas

> A good proof of work is difficult to calculate and easy to verify (i.e. an extremely low hash value). That's a specific Proof-of-Work system known as Hashcash [1]. There are many others, such as Cuckoo Cycle, in which the solution is a fixed-length cycle in a random bipartite graph [2]. [1] https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash [2] http://cryptorials.io/beyond-hashcash-proof-work-theres-mining-hashing/ http://cryptorials.io/beyond-hashcash-proof-work-theres-mini...