As I know, Ethereum Casper PoS implementation randomly selects 250 validators from the validator poll for an epoch. What is the random algorithm for this selection?
I am not sure about casper but POS algortihms in general use two common methods which are Randomised Block Selection and Coin Age Selection.
In the first, validators are selected based on a combination of the lowest hash value and highest stakes. Whereas, in the Coin Age method, validators are chosen based on how long their tokens have been staked for.