Only 17% of all 64-bit Integers are products of two 32-bit integers

4 days ago (lemire.me)

> I find it interesting to consider that if you pick a value at random, it will usually fail! That is, most 64-bit integers cannot be written as the product of two 32-bit integers.

While I find the 17% number interesting to think about, "most" is far less interesting. Multiplication doesn't care about order so you're instantly cutting 2^64 possibilities down to about 2^63. That's a hair's breadth away from "most" already, and considering even a tiny amount of overlapping results gets you there.

What gets interesting is actually trying to quantify the overlapping results.

  • A lot of the remaining is multiples of 4, which you can either get from having a 2 in both factors or a 4 in one (multiples of 9 are similar).

  • ... or just considering the even numbers almost all of them are 2 x N where N>2^32 and that gets you to within a hair of "most" and if you add in the odd thirds for which the same is true you get a bound of 2/3 - epsilon.

This feels like a underlying property that contributes to of Benford's Law[0]. That is, most numbers we measure and record are the results of various independent (addition) and dependent (multiplication) factors stacking together, and we observe this property in the distribution of them.

[0]: https://en.wikipedia.org/wiki/Benford%27s_law

There are about 4 billion more 64 bit integers than 32 bit integers.

The chance of a random 64 bit integer being a 32 bit integer is 0.0000000233 %

The chance of a random 64 bit integer being a product of two 32 bit integers is 17%

Nice

At the upper end you burn through the dynamic range pretty quickly. Largest eight products of 8-bit fixed-point numbers:

  {255 through 226 not used}   15 \* 15 = 225 
  {224 through 211 not used}  15 \* 14 = 14 \* 15 = 210 
  {197 through 209 not used}  15 \* 13 = 13 \* 15 = 195    
  14 \* 14 = 196
  {183 through 194 not used}   14 \* 13 = 13 \* 14 = 182

Toy example. Of course with 32-bit x 32-bit products, you skip way more 64-bit numbers than shown above.

I dream of a future where all 64-bit integers are products of 32-bit integers. Together, we can change math for the better.

  • I upvoted you, not because I think your joke is particularly great, but I hate that HN has this tendency to downvote comments that are clearly meant as a humorous contribution. And I get it, no-one wants HN to turn into Reddit. I also understand that not every joke lands. But I just think it's unnecessary to downvote, you could simply ignore.

    • "Ignore" is one of those things that sounds like it's a neutral choice but really isn't in practice - it's still just saying "can only ever be positively pressured". IMO people shouldn't go as far as flag though, at the very least, and if it's already at the bottom of the sort there is no sense dumping on it further.

      My current comment itself, for instance, also doesn't really add anything to the discussion about the article and I'd have no expectation people leave it from going negative. Maybe the will, maybe they won't, but there is no reason to expect they should in principle of me loving tangents :D.

If this seems counterintuitive, consider that only about a third of the two-digit numbers ({0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 25, 27, 28, 30, 32, 35, 36, 40, 42, 45, 48, 49, 54, 56, 63, 64, 72, 81}) can be written as the product of two one-digit numbers.