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top ten coolest numbers
This is an attempt to give a countdown of the top ten coolest numbers. Let’s first admit that this is a highly subjective ordering–one person’s 14.38 is another’s $\frac{\pi^{2}}{6}$. The astute (or probably simply “awake”) reader will notice, for example, a definite bias toward numbers interesting to a number theorist in the below list. (On the other hand, who better to gauge the coolness of numbers than a numbertheorist…) But who knows? Maybe I can be convinced that I’ve left something out, or that my ordering should be switched in some cases. But let’s first set down some ground rules.
What’s in the list? What makes a number cool? I think a word that sums up the key characteristic of cool numbers is “canonicity”. Numbers that appear in this list should be somehow fundamental to the nature of mathematics. They could represent a fundamental fact or theorem of mathematics, be the first instance of an amazing class of numbers, be omnipresent in modern mathematics, or simply have an eerily long list of interesting properties. Perhaps a more appropriate question to ask is the following:
What’s not in the list? There are some really awesome numbers that I didn’t include in the list. I’ll go through several examples to get a feel for what sorts of numbers don’t fit the characteristics mentioned above.
Shocking as it may seem, I first disqualify the constants appearing in Euler’s formula $e^{{i\pi}}+1=0$. This was a tough decision. Perhaps these five ($e$, $i$, $\pi$, 1, and 0) belong at the top of the list, or perhaps they’re just too fundamentally important to be considered exceptionally cool. Or maybe they’re just so cliché’d that we’ll get a significantly more interesting list by excluding them.
Also disqualified are numbers whose primary significance is cultural, rather than mathematical: despite being the answer to life, the universe, and everything, 42 is a comparatively uninteresting number. Similarly not included in the list were 8765309, 666, and the first illegal prime number. Similarly disqualified were constants of nature like Newton’s $g$ and $G$, the fine structure constant, Avogadro’s number, etc.
Finally, I disqualified number that were highly noncanonical in construction. For example, the prime constant and Champoleon’s constant are both mathematically interesting, but only because they were, at least in an admittedly vague sense, constructed to be as such. Also along these lines are numbers like G63 and Skewe’s constant, which while mathematically interesting because of roles they’ve played in proofs, are not inherently interesting in and of themselves.
That said, I felt free to ignore any of these disqualifications when I
felt like it. I hope you enjoy the following list, and I welcome
feedback.
Honorable Mentions

65,537  This number is arguably the number with the most potential. It’s currently the largest Fermat prime known. If it turns out to be the largest Fermat prime, it might earn itself a place on the list, by virtue of thus also being the largest odd value of $n$ for which an $n$gon is constructible using only a rule and compass.

Conway’s constant  The construction of the number can be found here http://mathworld.wolfram.com/ConwaysConstant.html. Though this number has some remarkable properties (not the least of which is being unexpectedly algebraic), it’s completely noncanonical construction kept it from overtaking any of our list’s current members.

1728 and 1729  This pair just didn’t have quite enough going for them to make it. 1728 is an important $j$invariant of elliptic curves and modular forms, and is a perfect cube. 1729 happens to be the third Carmichael number, but the primary motivation for including 1729 is because of the mathematical folklore associated it to being the first taxicab number, making it more interesting (math)historically than mathematically.

28  Aside from being a perfect number, a fairly interesting fact in and of itself, the number 28 has some extra interesting “aliquot” properties that propels it beyond other perfect numbers. Specifically, the largest known collection of sociable numbers has cardinality 28, and though this might seem a silly feat in and of itself, the fact that sociable numbers and perfect numbers are so closely related may reveal something slightly more profound about 28 than it just being perfect.
And now, on to the top 10:
#10) The Golden Ratio, $\phi$
This was a tough
one. Yes, it’s cool that it satisfies the property that its
reciprocal is one less than it, but this merely reflects that it’s a
root of the wholly generic polynomial $x^{2}x1=0$. Yes, it’s cool
that it may have an aesthetic quality revered by the Greeks, but this
is void from consideration for being nonmathematical. Only slightly
less canonical is that it gives the limiting ratio of subsequent
Fibonacci numbers. Redeeming it, however, is that this generalizes to
all “Fibonaccilike” sequences, and is the solution to two
sort of canonical operations:
$\displaystyle\frac{1}{1+\frac{1}{1+\frac{1}{1+\frac{1}{\ddots}}}}$ 
and
$\displaystyle\sqrt{1+\sqrt{1+\sqrt{1+\cdots}}}$ 
Also, this number plays an important role in the hstory of algebraic number theory. The field it generates is the first known example of a field in which unique factorization fails. Trying to come to grips with this fact led to the invention of ideal theory, class nubers, etc.
#9) 691
The prime number 691 made it on here for
a couple of reasons: First, it’s prime, but more importantly, it’s the
first example of an irregular prime, a class of primes of
immense importance in algebraic number theory. (A word of caution:
it’s not the smallest irregular prime, but it’s the one that
corresponds to the earliest Bernoulli number, $B_{{12}}$, so 691 is only
“first” in that sense). It also shows up as a coefficient of every
nonconstant term in the $q$expansion of the modular form
$E_{{12}}(z)$. Further testimony to the arithmetic significance is its
seemingly magical appearance in the algebraic $K$theory: It’s known
that $K_{{22}}(\mathbb{Z})$ surjects onto 691.
#8) 78,557
The number 78,557 is here to represent
an amazing class of numbers called Sierpinski numbers, defined
to be numbers $k$ such that $k2^{n}+1$ is composite for every
$n\geq 1$. That such numbers exist is flabbergasting…we know from
Dirichlet’s theorem that primes occur infinitely often in nontrivial
arithmetic sequences. Though the sequence formed by $78557\cdot 2^{n}+1$ isn’t arithmetic, it certainly doesn’t behave multiplicatively
either, and there’s no apparent reason why there shouldn’t be a large
(or infinite) number of primes in every such sequence. This
notwithstanding, Sierpinski’s composite number theorem proves there
are in fact infinitely many odd such numbers $k$. As a small
disclaimer, though it’s proven that 78,557 is indeed a Sierpinski
number, it is not quite yet known that it is the smallest. There are
17 positive integers smaller than 78,557 not yet known to be
nonSierpinski.
#7) $\frac{\pi^{2}}{6}$
Perhaps the first striking
this about this number is that it is the sum of the reciprocals of the
positive integer squares:
$\displaystyle 1+\frac{1}{4}+\frac{1}{9}+\cdots+\frac{1}{n^{2}}+\cdots=\frac{% \pi^{2}}{6}.$ 
Though the choice of $2$ here for the exponent is somewhat noncanonical (i.e. we’ve just noted that $\zeta(2)=\frac{\pi^{2}}{6}$, where $\zeta$ stands for the Riemann zeta function), and that this is largely interesting for mathhistorical reasons (it was the first sum of this type that Euler computed), we can at least include it here to represent the amazing array of numbers of the form $\zeta(n)$ for $n$ a positive integer. This class of numbers incorporates two amazing and seemingly disparate collections, depending on whether $n$ is even (in which case $\zeta(n)$ is known to be a rational multiple of $\pi^{n}$) or odd (in which case extremely little is known, even for $\zeta(3)$.
Further, there’s something slightly more canonical about the fact that its reciprocal, $\frac{6}{\pi^{2}}$, gives the “probability” (in a suitablydefined sense) that two randomly chosen positive integers are relatively prime.
#6) Feigenbaum’s constant
 This is the entry on
this the list with which I have the least familiarity. The one thing
going for it is that it seems to be highly canonical, representing
the limiting ratio of distance between bifurcation intervals for a
fairly large class of onedimensional maps. In other words, all maps
that fall in to this category will bifurcate at the same rate, giving
us a glimpse of order in the realm of chaotic systems.
#5) 2
This number caused quite a bit of
controversy in discussions leading up to the construction of this list.
The question here is canonicality. The first argument of “It’s the
only even prime” is merely a rewording of “It’s the only prime
divisible by 2,” which could uniquely characterizes any prime
(e.g. 5 is the only prime divisible by 5, etc.). Of debatable
canonicality is the immensely prevalent notion of “working in
binary.” To a computer scientist, this may seem extremely canonical,
but to a mathematician, it may simply be an (not quite) arbitrary
choice of a finite field over which to work.
Yet 2 has some remarkable features even ignoring aspects relating to its primality. For instance, the somewhat canonical field of real numbers $\mathbb{R}$ has index 2 in its algebraic closure $\mathbb{C}$. The factor $2\pi i$ is prevalent enough in complex and Fourier analysis that I’ve heard people lament that $\pi$ should have been defined to be twice its current value. It’s also the only prime number $p$ such that $x^{p}+y^{p}=z^{p}$ has any rational solutions.
Finally, if nothing else, it is certainly the first prime, and could at least be included for being the first representative of such an amazing class of numbers.
#4) 808017424794512875886459904961710757005754368$\times 10^{9}$
The above integer is the size of the monster group, the largest of the sporadic groups. This gives it a relatively high degree of canonicality. It’s unclear (at least to me) why there should be any sporadic groups, or why, given that they exist, there should only be finitely many. Since there is, however, there must be something fairly special about the largest possible one.
Also contributing to this number’s rank on this list is the remarkable properties of the monster group itself, which has been realized (actually, was constructed as) a group of rotations in 196,883dimensional space, representing in some sense a limit to the amount of symmetry such a space can possess.
#3) EulerMascheroni Constant, $\gamma$
One of
the most amazing facts from elementary calculus is that the harmonic
series diverges, but that if you put an exponent on the denominators
even just a hair above 1, the result is a convergent sequence.
A refined statement says that the partial sums of the harmonic series
grow like $\ln(n)$, and a further refinement says that the error of
this approximation approaches our constant:
$\displaystyle\lim\limits_{{n\rightarrow\infty}}1+\frac{1}{2}+\frac{1}{3}+% \cdots+\frac{1}{n}\ln(n)=\gamma.$ 
This seems to represent something fundamental about the harmonic series, and thus of the integers themselves.
Finally, perhaps due to importance inherited from the crucially important harmonic series, the EulerMascheroni constant appears magically all over mathematics.
#2) Khinchin’s constant, $K\approx 2.685452...$
For a real number $x$, we define a geometric mean function
$\displaystyle f(x)=\lim\limits_{{n\rightarrow\infty}}(a_{1}\cdots a_{n})^{{1/n% }},$ 
where the $a_{i}$ are the terms of the simple continued fraction expansion of $x$. By nothing short of a miracle of mathematics, this function of $x$ is almost everywhere (i.e. everywhere except for a set of measure 0) independent of $x$!!! In other words, except for a “small” number of exceptions, this function $f(x)$ always outputs the same value, which is called Khinchin’s constant and is denoted by $K$. It’s hard to impress upon a casual reader just how astounding this is, but consider the following: Any infinite collection of nonnegative integers $a_{0},a_{1},\ldots$ forms a continued fraction, and indeed each continued fraction gives an infinite collection of that form. That the partial geometric means of these sequences is almost everywhere constant tells us a great deal about the distribution of sequences showing up as continued fraction sequences, in turn revealing something very fundamental about the structure of real numbers.
#1) 163
Well, we’ve come down to it, this
author’s humble opinion of the coolest number in existence. Though an
unlikely candidate, I hope to show you that 163 satisfies so many
eerily related properties as to earn this title.
I’ll begin with something that most number theorists already know about this number – it is the largest value of $d$ such that the number field $\mathbb{Q}(\sqrt{d})$ has class number 1, meaning that its ring of integers is a unique factorization domain. The issue of factorization in quadratic fields, and of number fields in general, is one of the principal driving forces of algebraic number theory, and to be able to pinpoint the end of perfect factorization in the quadratic case like this seems at least arguably fundamental.
But even if you don’t care about factorization in number fields, the above fact has some amazing repercussions to more basic number theory. The two following facts in particular jump out:

$e^{{\pi\sqrt{163}}}$ is within $10^{{12}}$ of an integer.

The polynomial $f(x)=x^{2}+x+41$ has the property that for integers $1\leq x\leq 41$, $f(x)$ is prime.
Both of these are tied intimately (the former using deep properties of the $j$function, the latter using relatively simple arguments concerning the splitting of primes in number fields) to the above quadratic imaginary number field having class number 1. Further, since $\mathbb{Q}(\sqrt{d})$ is the last such field, the two listed properties are in some sense the best possible.
Most striking to me, however, is the amazing frequency with which 163 shows up in a wide variety of class number problems. In addition to being the last value of $d$ such that $\mathbb{Q}(\sqrt{d})$ has class number 1, it is the first value of $p$ such that $\mathbb{Q}(\zeta_{p}+\zeta_{p}^{{1}})$ (the maximal real subfield of the $p$th cyclotomic field) has class number greater than 1. That 163 appears as the last instance of a quadratic field having unique factorization, and the first instance of a real cyclotomic field not having unique factorization, seems too remarkable to be coincidental. This is (maybe) further substantiated by a couple of other factoids

Hasse asked for an example of a prime and an extension such that the prime splits completely into divisors which do not lie in a cyclic subgroup of the class group. The first such example is any prime less than 163 which splits completely in the cubic field generated by the polynomial $x^{3}=11x^{2}+14x+1$. This field has discriminant $163^{2}$. (See Shanks’ The Simplest Cubic Fields).

The maximal conductor if an imaginary abelian number field of class number 1 corresponds to the field $\mathbb{Q}(\sqrt{67},\sqrt{163})$, which has conductor $10921=67*163$.
It is unclear whether or not these additional arithmetical properties reflect deeper properties of the $j$function or other modular forms, and remains a wide open field of study.
Originally posted on Cam’s homepage
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Comments
paradox
I think that the coolest number is the first uncool number :)
2 is the oddest of all primes
I completely agree with your choice of making 2 apear in your list. It has been said many times that "2 is a very odd prime".
Another reason you may want to add to your endorsement for 2 is that x^p+y^p=z^p has solutions for p=2 but not for any other prime! (i.e. FLT). This is actually quite surprising and singles out the prime 2.
Alvaro
Re: 2 is the oddest of all primes
Excellent point. I'll add that immediately.
Cam
6 is a cool number
Because 1+2+3=1.2.3 and there is not another one with that property!
Re: paradox
In fact we could say that the first uncool number is 0 because I cannot divide any number (except, possibly, the same one)by him. What a subjectiveness!
Re: 6 is a cool number
However, 4 is cooler:
4 = 2 + 2 = 2*2 = 2^2... and it's the only one with THAT property.

"It's John Ashcroft's vision of hell. It's a Tom Waits song."
Simon  mhm27x5
Re: 6 is a cool number
Why not? That's your opinion. I like the six because 1,2,3 are the proper divisors of 6. And that's the point about the mathematical context of ``cool number''. I think that mathcam entry has to do with curious/notable numbers. However rspuzio, apparently, through 'paradox' tries to give a logical/mathematical sense to the mentioned phrase (or it just was a joke?).
Re: paradox
I think that the first uncool number is the last uncool number... Let's find it...
Is this math?
Re: 6 is a cool number
6 is unique also being the least order giving a noncommutative group =o)
Re: 4 is a cool number
4 is unique also in having the property that the least noncommutative ring, the "Klein 4ring", consists of four elements =o)
Re: 4 is a cool number
A_4 is also the only nonsimple alternating group.
4 is also unique in that R^n has a unique differentiable structure for all n... except 4.

"It's John Ashcroft's vision of hell. It's a Tom Waits song."
Simon  mhm27x5
Re: 4 is a cool number
> 4 is also unique in that R^n has a unique differentiable
> structure for all n... except 4.
Is that right? I thought it was just the smallest n with that property. I actually thought about putting 4 on the list because of that, but if it's *unique* in having that property, it might at least earn 4 an honorable mention.
Cam
Re: 4 is a cool number
I'm pretty sure it's unique... at least every time I've heard that mentioned, it's always been said that way.
If that is the case though, it (at least to me) puts 4 in a pretty special category. All the other little things I mentioned are curiosities (and the A_4 bit is, admittedly, somewhat artificial)... but the differentiable structure thing... that's interesting.

"It's John Ashcroft's vision of hell. It's a Tom Waits song."
Simon
If you want an amusing read...
...the "Ten Coolest Numbers" article just got Dugg.
The ensuing discussion is rather humorous.
http://digg.com/general_sciences/The_Ten_Coolest_Numbers
Cam