The invention of zero as a real number (India, ~5th century). Not just a placeholder digit, but a number you could actually compute with - it's the hinge that our entire positional number system and modern arithmetic swing on. is leading so far with 25%.
Cantor's proof that infinities have different sizes (1874). He showed there are strictly more real numbers than whole numbers, even though both sets are infinite. The idea was so radical that fellow mathematicians savaged him for it, yet it's now the bedrock of modern set theory. is second. On this few votes the split is still anyone's guess.
Everyone who has answered so far is voting from China.
The poll is still live and picked up a vote today.
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