Equihash (Birthday Problem): Memory Hardness https://en.m.wikipedia.org/wiki/Equihash https://en.m.wikipedia.org/wiki/Equihash RandomX (Execution of a random p

Equihash (Birthday Problem): Memory Hardness https://en.m.wikipedia.org/wiki/Equihash https://en.m.wikipedia.org/wiki/Equihash RandomX (Execution of a random program): Memory Hardness (Inc. cache sizes), Speculative Execution/Branching, ILP, some sort of chaining https://github.com/tevador/RandomX/blob/master/doc/design.md https://github.com/tevador/RandomX/blob/master/doc/design.md Edit: these are examples of CPU-bound PoW. But the general idea with PoW is that you have some hash-like function H() with no known inverse function such that the only feasible way to determine the output is just running the function. The client runs H(x) with a different input x every time. If the output is a high enough number, the server lets the client though. The server runs H() to verify, and this is easy to parallelize. But in order to get through the server, the client must run H() many times on average. Also server provides a salt to prevent the client from reusing their old hashes. And the server usually indicates how high the output of H() must be (this is called the difficulty).