In case it’s of interest - Gapcoin’s [1] Proof of Work function arguably does useful work by searching for large prime gaps of record merit ... “As of December 2017, the largest known merit value and first with merit over 40, as discovered by the Gapcoin network, is 41.93878373 with the 87-digit prime 293703234068022590158723766104419463425709075574811762098588798217895728858676728143227. The prime gap between it and the next prime is 8350” [2]
[1] https://gapcoin-project.github.io/
[2] https://en.wikipedia.org/wiki/Prime_gap
The research is undermined somewhat by the omission of Namecoin[1], launched back in 2013, still being developed [2] and ironically, listed on coingecko [3].
fwiw: https://github.com/DOACC documented at https://doacc.github.io/
An historical snapshot (in RDF) of the metadata of ~3500 altcoins from mid-2014 to early 2017 when the recording effort was abandoned.