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The article didn't explain the abc conjecture in much detail ("The original proof is of the long-standing ABC conjecture that explores the deep nature of number
by lvh 9y ago
The article didn't explain the abc conjecture in much detail ("The original proof is of the long-standing ABC conjecture that explores the deep nature of numbers, centred on the simple equation a + b = c."). That's unfortunate, because even though the proof is inscrutable, the conjecture itself isn't very complicated:
If three numbers a, b, and c are coprime and two numbers a and b have large prime factors, then their sum c generally does not.
To make that a little more formal, it means that the radical (product of distinct prime factors) of their product abc is usually less than the sum c, up to some exponent 1+eps:
rad(abc)^(1+eps) < c
... for some eps. That eps is important! There are infinitely many numbers a, b, c where this isn't true for some eps (and there are ways to construct those), but the conjecture states that for any given eps, there are finitely many.
To find maximal counterexamples (which, because mathematics consistently uses intuitive terminology, are said to be "high-quality") you typically look for numbers that are smooth (small prime factors) but not powersmooth (larger prime-factors-raised-to-a-power). Typically you'll also fix a to be really small, to limit the search space somewhat. [MM14] describes other methods for finding triples.
Here's a simple example of a high-quality triple:
a = 1
b = 2*3^7 = 4374
c = 5^4*7 = 4375
Because all factors are unique, rad(abc) = rad(a) rad(b) rad(c).
c > (rad(abc))^(1+eps)
c > (rad(a) * rad(b) * rad(c))^(1+eps)
5^4*7 > (rad(1) * rad(2*3^7) * rad(5^4*7))^(1+eps)
5^4*7 > (1 * 2 * 3 * 5 * 7)^(1+eps)
4375 > (210)^(1+eps)
This is a great example, because the exponent (sometimes called q) is about 1.57, which is extraordinarily high. In the first billion c's, there are only 34 with q > 1.4. This is the fifth best such triple we know of. [triples]
Consider a trivial case where all three numbers are prime; a = 2, b = 3, c = 5. Primes are their own radical, so:
5 > (2 + 3)^1+eps
This is the edge case.
[triples]: https://www.math.leidenuniv.nl/~desmit/abc/index.php?set=2 https://www.math.leidenuniv.nl/~desmit/abc/index.php?set=2
[MM14]: https://arxiv.org/pdf/1409.2974.pdf https://arxiv.org/pdf/1409.2974.pdf
- Koshkin 9y agoHere's a link to the best quick and elementary (yet detailed) explanation of what ABC conjecture is all about: https://www.youtube.com/watch?v=XYisYYhKKYA https://www.youtube.com/watch?v=XYisYYhKKYA.
- lvh 9y agoI can no longer edit this comment, but there's a typo at the bottom: 5 > (2 + 3)^1+eps ... should be 5 > (2 * 3 * 5)^1+eps Which is a good example of the "common case", i.e. not a triple of particularly high quality (this is q=0.4-something).
- legel 9y agoThanks for the edit, I was wondering how that would make sense otherwise. More generally, thanks for clearly communicating what's really going on! Math can only be extremely beautiful if it is actually understood.
- dmix 9y ago> Math can only be extremely beautiful if it is actually understood. This is why I've found relearning math very rewarding. Even if I haven't found as many applications in my day-to-day programming life as I hoped (other than maybe when writing Haskell-esque languages or when I'm fortunate to get to use pure FP concepts).