

An attempt at the world's biggest math magic trick. - RiderOfGiraffes
http://pidaymagic.com/

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pbhjpbhj
"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.

~~~
RiderOfGiraffes
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.

~~~
pbhjpbhj
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.

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

~~~
pbhjpbhj
Yes.

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

~~~
RiderOfGiraffes
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.

~~~
fh
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.

~~~
RiderOfGiraffes
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.

------
RiderOfGiraffes
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.

~~~
jacquesm
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.

~~~
RiderOfGiraffes
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.

~~~
jacquesm
Just posting a link is asking for an upvote :)

~~~
RiderOfGiraffes
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.

~~~
RiderOfGiraffes
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.

