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the smallest number that looks prime but isn't
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redbaker
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Sun Dec 23 2007 at 20:00:54
No joke
— there's a semi-serious proof involved here. We're looking for the smallest number that could easily be mistaken for
prime
but in fact is not. How do we find it? Well, since it's not prime, let's look for its
prime factors
.
The number can't be a multiple of
two
—
even
numbers are too easy to spot.
The number can't be a multiple of
three
— there's an
easy test
for that.
The number can't be a multiple of
five
— thanks to our
base ten
number system, it's too easy to find those.
Seven
? Sure, why not? Multiples of seven don't look special at all. But we need more than one prime factor -- everybody knows that
49
is seven
squared
.
The other factor can't be
eleven
— 7 x 11 = 77, obviously not prime. Multiples of 11, especially low ones, are fairly obvious.
But what about
thirteen
? 7 x 13 = 91. That...
looks prime
.
So there you have it. A
rigorous
proof
that the smallest number that looks prime but isn't is
91
. Use this to impress your friends and shame your enemies at
cocktail parties
.
But what about
the smallest number that can't be described in fewer than 15 words
?
printable version
chaos
Richard Paradox
the smallest integer never considered
prime number
You have far too much time on your hands
Prime decomposition
Even
The voting system doesn't work because I'm drunk and that fucks it up
The only thing stopping us is the pressure. If your machine can withstand it, we are already on our way.
countable
E2 Secret Santa 2007
Notelet
The Unreasonable Effectiveness of Mathematics
Methods of mathematical proof
base ten
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