Canada’s Central Bank (BOC) released a working paper this month, Incentive Compatibility on the Blockchain. In it, BOC seems to argue the world’s most popular cryptocurrency, Bitcoin, is safe from a 51% attack, double spend scenario through the right incentive structure and proof of work (POW).
Also read: http://cryptowizard.biz/canada-central-bank-bitcoin-safe-from-51-attack-double-spending/