Researchers Claim Bitcoin’s Blockchain Can Be Secured Even if 51% of Miners Are MaliciousOne of the fundamental assumptions of blockchains, whether public or private, is that 51% of actors, especially miners, are honest. Logically, the more actors there are, the more the assumption stands. However, numerous bitcoin developers, including Gregory Maxwell, Peter Todd, Luke-Jr and others, have stated that bitcoin mining is centralized in two or three individuals.
https://www.cryptocoinsnews.com/researchers-bitcoin-blockchain-secured-inspite-of-51-attack/