  We consider mechanisms for truthfully eliciting probabilistic predictions
from a group of experts. The standard approach -- using a proper scoring rule
to separately reward each expert -- is not robust to collusion: experts may
collude to misreport their beliefs in a way that guarantees them a larger total
reward no matter the eventual outcome. Chun and Shachter (2011) termed any such
collusion "arbitrage" and asked whether there is any truthful elicitation
mechanism that makes arbitrage impossible. We resolve this question positively,
exhibiting a class of strictly proper arbitrage-free contract functions. These
contract functions have two parts: one ensures that the total reward of a
coalition of experts depends only on the average of their reports; the other
ensures that changing this average report hurts the experts under at least one
outcome.
