Securing a public ledger is useful. The only kinds of problems that are suitable for PoW are those with predictable computational cost. The only thing that eve

Securing a public ledger is useful. The only kinds of problems that are suitable for PoW are those with predictable computational cost. The only thing that even approaches this and is also extrinsically useful is maybe math problems like finding prime numbers, but we don’t actually know what complexity class that falls under.