Sorry for galvanizing the corpse, don't want to create a new topic.

Nope, the 51% is true only for two-miners network.

For more complex configuration, supposing the finding bock events are completely independent and the probability is only proportional to the hashrate.

Consider a [relatively simple] minig network where hashrates are divided as 40:3x20 %%.

Then, Probability of at least one of "weak" miners would mine a block is 1 minus product of probabilities of the inverse event:

https://www.khanacademy.org/math/ap-statistics/probability-ap/probability-multiplication-rule/a/general-multiplication-ruleThus, Pr = 1 - 0.8*0.8*0.8 = 1 - 0.512 = 48.8%

Hence, [for given above configuration] one need only 40% of the network hashrate to gain 51.2% probability to mine a block, that means in long therms his fork will eventually be longer.

If, for example, we consider 7x10% and handful of 1%-hashrate-miners - then we need even less, slightly above 30% of total hashrate to overtake the 50% probability.

BTW, according the formula above, the swarm of micro-miners DOES NOT contribute to decentralization but vice versa, decrease the threshold for this kind of "attack".