# Number Gossip

(Enter a number and I'll tell you everything you wanted to know about it but were afraid to ask.)

## Unique Properties of 8

- 8 is the only composite cube in the Fibonacci sequence
- 8 is the dimension of the octonions and is the highest possible dimension of a normed division algebra
- 8 is the smallest number (except 1) which is equal to the sum of the digits of its cube
- "Eight" is the lexicographically first string representing an integer in English
- 8 is the sum of two first two odd primes
- 8 is the largest Fibonacci number with a prime neighbor
- the smallest number of acute triangle a square can be triangulated into is 8
- The smallest number of perfect riffle out-shuffles to return a standard deck of cards to its original order is 8

## Rare Properties of 8

The n-th *cake* number is the maximum number of pieces a (cylindrical) cake can be cut into with n (straight-plane) cuts.

Unfortunately, not everybody gets the frosting. If you cut pizza rather than cake, you get lazy caterer's numbers.

The number n is a *cube* if it is the cube of an integer.

*Fibonacci* numbers are numbers that form the Fibonacci sequence. The Fibonacci sequence is defined as starting with 1, 1 and then each next term is the sum of the two preceding ones.

Fibonacci numbers are very common in nature. For example, a pineapple has 8 spirals if you count one way, and 13 if you count the other way.

A k-digit number n is called *narcissistic* if it is equal to the sum of k-th powers of its digits. They are also called *Plus Perfect* numbers.

A number is a *power of 2* if it is 2 to some power.

## Common Properties of 8

A positive integer greater than 1 that is not prime is called *composite*.

Composite numbers are opposite to prime numbers.

The number n is *deficient* if the sum of all its positive divisors except itself is less than n.

Compare with perfect and abundant numbers.

A number is *even* if it is divisible by 2.

Numbers that are not even are odd. Compare with another pair -- evil and odious numbers.

The number n is *odious* if it has an odd number of 1's in its binary expansion.

Guess what evil numbers are.

A *palindrome* is a number that reads the same forward or backward.

An integer n is *powerful* if for every prime p dividing n, p^{2} also divides n.

How much power? They all can be written as a^{2} b^{3}.

The number n is *practical* if all numbers strictly less than n are sums of distinct divisors of n.

The next *Ulam* number is uniquely the sum of two earlier distinct Ulam numbers.