An attempt at the world's biggest math magic trick. | Hacker News Reader

An attempt at the world's biggest math magic trick.

pidaymagic.com

"This is not one of those lame mathematical tricks..."

Er, you keep telling yourselves that.

Also how is "13 Mar 10 / Welcome to Pi Day Magic: Sunday 14th March 2010" going to give anything but 14.3 - they forgot to non-standardise their onsite dates.

If you think it's not a lame mathematical trick, would you care to say how you think it's done? Or at least, say something that makes it clear to those who know how it's done that you also know the answer? Like, for example, tell us how it can go wrong.

Although I'm not actually involved in this event, it is the sort of thing that I follow closely, and on other occasions am involved in. I am especially interested to discover what people already know so I can avoid well-known, obvious or boring stuff, and produce things that are more challenging.

I suspect they couldn't do much with 0000000, where the first digit that is chopped off is my secret digit.

I was under the impression that this trick usually involves the difference of two numbers with the same digits.

Well Benford's law says it's more likely to be a 1 but the rest of it falls into the 5% failure rate they're allowing. Also from my skim of the rules I think you have one to many zeros.

To get it exactly right would be luck, they're going for 95% correct. Tables. They're limiting the search space considerably in the setup (I skimmed) such that it appears you're supplying random numbers to them but they're quite narrowly defined and all but one of the digits provides enough of a key to lookup the last digit.

For example in the range (1000000,1000040) only 1000000 can be produced from multiplying single digits; there are 4 primes in that range FWIW.

Add in some test data, and perhaps something like Benford's law and bobs your uncle.

FWIW, I'm not telling you it's obvious, nor that you have to think it's lame too.

Would it impress you more to know that I can this trick live and without assistance?

Yes.

ISO Dates are in the form yyyy-mm-dd. http://en.wikipedia.org/wiki/Iso_date

Um, yeah, I kinda know that. So Pi is 2.0100314 now?

The point was they're using USA date order as opposed to one of the more international conventions, as they use on their website. A petty point, but hey this is teh intarwebs.

I guess the trick is there aren't that many possibilities, so you precomputed them all can search through the list.

You can choose any collection of digits and multiply them together. Then you can choose any digit of the answer as your secret, and you can list the remaining digits in any order you like. Trust me, there are lots of possibilities.

No, that's not the trick.

I'm sure there's a cute mathematical trick to calculate the missing digit, but Raphael is right, with numbers limited to 1 billion, this can be easily brute forced. You need a few gigabytes of disk space and a few days of precalculation at most.

What's more interesting to me is that the missing number is actually unique, i.e. that this trick is possible at all. Maybe it actually isn't, maybe it doesn't work in 0.1% of the cases, which is good enough for a magic trick.

It works in very nearly (but not quite) 95% of cases.

I should also have said - this trick doesn't rely on the limit of the number of digits, it works with any number of digits. Use an arbitrary precision package or sensible language, multiply numbers together for as long as you like, then tell me all but one of the digits in the answer, and 95% of the time I can tell you the one you left out.

Perhaps. But I can do this live, without assistance.

OK, so many of you will know, or be able to work out, how this will work, but I've used exactly this trick to freak out so many people. Join in, tell others, help to raise the profile of math and science.

Please.

ADDED IN EDIT: Please up-vote this submission to get it to the front page. If you don't want me to get karma, feel free to down-vote this comment as a scapegoat. If it bottoms out I'll add another scapegoat. Thanks.

Interesting trick with the scapegoat system, but I think it fixes something that doesn't need fixing.

I can't imagine people that would even bother about you not getting your karma but to upvote the story anyway.

That would mean you think there are people that would withhold their vote simply because it is you that does the posting. I can't believe it's come to that.

People might resent being asked for an up-vote. I suspect that many people would read something, then move on, not especially interested. They might consider up-voting having been asked, but then feel that the person asking is just doing so for the karma. I want to make it clear that I'm asking to get the item noticed, because I think the item has value, and not because I want the karma.

There are items where I've used this idea and simply been down-voted without getting many up-votes at all. That's fine, I really, to some extent, don't care about the karma.

But occasionally I care about the item, and I want to encourage people to up-vote without them resenting being asked.

I hope that's clear - thanks for giving me the opportunity to expand on my intentions.

Just posting a link is asking for an upvote :)

I can see your point of view, and no doubt many agree with it, but not to me. To me, posting a link is offering something that people might think is interesting.

To put the later comments into context ...

The deleted comment was someone saying that "Everyone knows" that this is a flagrant attempt to get twitter followers, that it's spam, and they'd flagged it. Having down-voted and flagged, they then discover that they were wrong, and instead of apologising and retracting the comment, they simply deleted it, leaving, of course, the down-votes.

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