0 is the
additive identity.
1 is the
multiplicative identity.
2 is the only even
prime.
3 is the number of spatial dimensions we live in.
4 is the smallest number of colors sufficient to color all planar maps.
5 is the number of
Platonic solids.
6 is the smallest
perfect number.
7 is the smallest number of faces of a regular polygon that is not
constructible by straightedge and compass.
8 is the largest
cube in the
Fibonacci sequence.
9 is the maximum number of
cubes that are needed to sum to any positive integer.
10 is the base of our number system.
11 is the largest known
multiplicative persistence.
12 is the smallest
abundant number.
13 is the number of
Archimedian solids.
14 is the smallest number n with the property that there are no numbers
relatively prime to n smaller numbers.
15 is the smallest
composite number n with the property that there is only one
group of order n.
16 is the only number of the form x
y = y
x with x and y different integers.
17 is the number of
wallpaper groups.
18 is the only number that is twice the sum of its digits.
19 is the maximum number of 4
th powers needed to sum to any number.
20 is the number of
rooted trees with 6 vertices.
21 is the smallest number of distinct
squares needed to tile a
square.
