4 ms·

I once met a guy who could compute logarithms in his head, as well as exponentials (or antilogarithms, as he called them). He did that slowly but steadily, at

I once met a guy who could compute logarithms in his head, as well as exponentials (or antilogarithms, as he called them). He did that slowly but steadily, at about one decimal per second, to an arbitrary precision. He could start giving the answer before you finished reciting your question. That's actually easy with logarithms, because the logarithm is essentially the number of digits. As a byproduct, he could compute square roots easily, and also cubic roots and tenth roots. The easiest case for him, he said, because it was just "a shift". I cannot recall his name, he was a kind of "showman" that did his tricks to a large audience. From Colombia or Peru, I think. He was invited to our math department, and after his show, he explained in petit comité some of the secret sauce. We were a bit surprised to hear that he essentially memorized a large part of a table of logarithms. But maybe he was just trolling us.

acchow 4y ago
> As a byproduct, he could compute square roots easily, and also cubic roots and tenth roots. The easiest case for him, he said, because it was just "a shift". https://en.wikipedia.org/wiki/Shifting_nth_root_algorithm https://en.wikipedia.org/wiki/Shifting_nth_root_algorithm
com2kid 4y ago
I used to be able to do logarithms in my head, not to arbitrary precision though, I'd just get 2 or 3 decimal places. Square roots, same thing, take enough math classes and don't use a calculator to do the arithmetic, and after a few quarters you can get down to a couple of digits pretty easily.
- gumby 4y ago
  If you used a slide rule you got a feel for what the answer will be, so in that case this guy's skill might be less than a superpower and more like good intuition. More likely though that case would not apply as slide rules have been dead for half a century. I'm not old enough to have used a slide rule "professionally" (i.e. for work or even school) but I did use my dad's old engineering slide rule for a while in high school physics out of sheer orneriness and desire to be weird. Once I got used to it I often had the answer (to a decimal place or two) faster than my classmates using calculators, when doing problems as the teacher was working them out on the board in class. Of course I had some advantages they did not, for example I only needed one or two digits of significance for problems like that, and if I got the power of 10 wrong it was immediately obvious.
nestorD 4y ago
> We were a bit surprised to hear that he essentially memorized a large part of a table of logarithms. But maybe he was just trolling us. I am not familiar with that particular demonstration but having studied similar things, they are all a trade-off between doing computation cleverly in your mind and memorizing information. The more you memorize and the less you have to compute on the fly, increasing speed (step 1 of practicing that type of show is to learn mnemotechnic thoroughly).

acchow 4y ago

> As a byproduct, he could compute square roots easily, and also cubic roots and tenth roots. The easiest case for him, he said, because it was just "a shift". https://en.wikipedia.org/wiki/Shifting_nth_root_algorithm https://en.wikipedia.org/wiki/Shifting_nth_root_algorithm

com2kid 4y ago

I used to be able to do logarithms in my head, not to arbitrary precision though, I'd just get 2 or 3 decimal places. Square roots, same thing, take enough math classes and don't use a calculator to do the arithmetic, and after a few quarters you can get down to a couple of digits pretty easily.

gumby 4y ago
If you used a slide rule you got a feel for what the answer will be, so in that case this guy's skill might be less than a superpower and more like good intuition. More likely though that case would not apply as slide rules have been dead for half a century. I'm not old enough to have used a slide rule "professionally" (i.e. for work or even school) but I did use my dad's old engineering slide rule for a while in high school physics out of sheer orneriness and desire to be weird. Once I got used to it I often had the answer (to a decimal place or two) faster than my classmates using calculators, when doing problems as the teacher was working them out on the board in class. Of course I had some advantages they did not, for example I only needed one or two digits of significance for problems like that, and if I got the power of 10 wrong it was immediately obvious.

nestorD 4y ago

> We were a bit surprised to hear that he essentially memorized a large part of a table of logarithms. But maybe he was just trolling us. I am not familiar with that particular demonstration but having studied similar things, they are all a trade-off between doing computation cleverly in your mind and memorizing information. The more you memorize and the less you have to compute on the fly, increasing speed (step 1 of practicing that type of show is to learn mnemotechnic thoroughly).