Primecoin (Cunningham chains) Gapcoin (finding large gaps between successive primes) Riecoin (finding maximally dense prime clusters of size 6) Nexus (findin

Primecoin (Cunningham chains) Gapcoin (finding large gaps between successive primes) Riecoin (finding maximally dense prime clusters of size 6) Nexus (finding almost-dense clusters with a maximum spacing between successive primes) As an aside, picking a mathematically interesting and intricate proof of work function is probably a bad idea, because someone like me will come along and optimize the miner and mine privately at a large profit margin, as I did with two of these coins.