What is the oracle problem?
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3382343
https://steemit.com/gridcoin/@jamescowens/gridcoin-leisure-update-4-0-3-0-released
https://medium.com/@kenchangh/keeping-honest-solving-the-oracle-problem-714addc06e53
https://hackernoon.com/a-discussion-of-the-oracle-problem-6cbec7872c10
https://medium.com/@antsankov/the-oracle-problem-isnt-a-problem-and-why-smart-contracts-makes-insurance-better-for-everyone-8c979f09851c
https://sci.smithandcrown.com/research/oracle-problem-and-mixicles