Wow, crazy randomly seeing my Algo prof at the top of HN winning this! Congrats Eshan!!
bhasi · 1h ago
Eshan's Bachelor's thesis advisor from IIT Kanpur, Prof Manindra Agrawal, also won the Gödel prize in 2006. Wow.
antics · 2h ago
If I give you a biased coin can you simulate a truly random coin flip with it? The answer turns out to be yes. Flip the biased coin twice: HT = heads, TH = tails, and HH/TT = flip twice again.
The general study of such things is called “randomness extractors”. The new Gödel prize goes to this paper which shows how to extract a nearly perfect random bit out of two sources with low min-entropy.
falcor84 · 2h ago
Yes, but - you need to replace "twice" there with "an unbounded number of times". If you apply this in an environment where the biased coin is coming from an external source, your system becomes susceptible to DoS attacks.
antics · 1h ago
While I obviously think randomness extractors over adversarial sources are very interesting, I think talking about them specifically in this example complicates the point I'm trying to make, which is that it's incredible it can be done at all.
dataflow · 1h ago
Note that adversarial is kind of a red herring, not sure why they mentioned that. The number of flips is unbounded regardless. Which is why it's not really incredible that it can be done: it can't, not as the problem was originally stated. What can be done is solving a different (but useful) problem than the one originally posed.
I realize this sounds like a minor detail to someone who finds this cool (and so do I), but I don't think it is. It's kind of frustrating to be told that your intuition is wrong by someone smarter than you, when your intuition is actually correct and the experts are moving the goalposts. IMO, it makes people lose respect for experts/authority.
antics · 47m ago
So, the problem in its original framing is: can we simulate a fair coin flip with an unfair coin? As stated, I do actually think the von Neumann response answer ("this is actually technically possible") is fair, in that if I wanted a solution in O(1), I think I should have to say so ahead of time.
I suppose we'll have to disagree about whether this is incredible. The response shows that (1) this can be done at all, and (2) that the answer is exponentially likely as time goes on, not asymptotically, but for finite n. Incredible! You don't see finite-decay bounds very often! If you don't think that's incredible I invite you to ask a room full of people, even with the qualifications you deem appropriate, e.g., "solution does not need to be constant-time", or whatever.
QuesnayJr · 51m ago
I have the exact opposite reaction, that if someone told me the answer is "no" because it requires an unbounded number of coin flips that they were the ones trying to bullshit me. In antic's formulation, nothing is said about requiring a bounded number of flips.
dataflow · 47m ago
"Simulate a truly random coin" implies it IMO. You're not simulating a truly random coin if you need unbounded time for a single flip. The truly random coin definitely doesn't need that. It just feels like a scam if someone sold me such a machine with that description - I'd want my money back. I don't expect everyone would feel the same, but I think a lot of people would.
The general study of such things is called “randomness extractors”. The new Gödel prize goes to this paper which shows how to extract a nearly perfect random bit out of two sources with low min-entropy.
I realize this sounds like a minor detail to someone who finds this cool (and so do I), but I don't think it is. It's kind of frustrating to be told that your intuition is wrong by someone smarter than you, when your intuition is actually correct and the experts are moving the goalposts. IMO, it makes people lose respect for experts/authority.
I suppose we'll have to disagree about whether this is incredible. The response shows that (1) this can be done at all, and (2) that the answer is exponentially likely as time goes on, not asymptotically, but for finite n. Incredible! You don't see finite-decay bounds very often! If you don't think that's incredible I invite you to ask a room full of people, even with the qualifications you deem appropriate, e.g., "solution does not need to be constant-time", or whatever.