Some related work for Hyperchains. Keep adding them in this thread.
Blog post: Hyperchains — secure, cheap & scalable blockchain technology for everyone
Yani’s original blog post where the concept is introduced.
Blog post: Understanding Proof of Stake through its flaws.
A three-part article. Part 2 discusses Nothing-at-Stake, and part 3 discusses long-range attacks and Weak Subjectivity.