Goldcoin was the first to introduce a 51% attack defense in 2013.
I found the marketing paper (It's not a whitepaper, and definitely not an academic paper) that claims to have solved the problem. There is
a thread in this forum about it which explains how it fails to solve anything and instead serves to expose the incompetence of those developers.
If you read the entire thread, Akumaburn does try his best to explain how and why this time-tested system works.