## October 21, 2005

### A different representation for integers

[Math_] The fundamental theorem of arithmetic states that every positive integer has a unique factorization into primes. Instead of the usual decimal notation where each position represents a multiple of powers of ten, we can represent each integer with a "primal" notation where each position is the power of a prime number. Here is what I mean:

2: (1)
3: (1 0)
4: (2)
5: (1 0 0)
6: (1 1)
7: (1 0 0 0)
8: (3)
9: (2 0)
10: (1 0 1)
11: (1 0 0 0 0)
12: (1 2)

The right-most position is power of two's, next to it is power of three's and so on. The intuition one gets from such a representation is remarkable. Let's prove a simple bound related to the prime number theorem (i.e. how many primes are there less than a given number?)

Let p(n) be the number of primes less than n where 2^k <= n < 2^(k+1). Now consider all the numbers smaller than n. Their primal representations are all distinct, can have at most p(n) elements, and none of the elements can be greater than k. This gives us the inequality:

k^p > n
p lg(k) > lg(n)
p > lg(n) / lg(lg(n))

Not a very tight bound, but could be used as an alternative to Euclid's proof of the infinity of the number of primes :)

One thing about our primal representation is bothering me. Namely the elements themselves are integers, but we are denoting them with regular decimal digits instead of the primal representation! The primal representation for 1 should be (). Zero is more of a problem. Let's keep it as 0 for now and see what we get:

1: ()
2: (())
3: (() 0)
4: ((()))
5: (() 0 0)
6: (() ())
7: (() 0 0 0)
8: ((() 0))
9: ((()) 0)
10: (() 0 ())
11: (() 0 0 0 0)
12: (() (()))

Much better. This representation gives us a one-to-one mapping between all integers and a certain subset of binary trees or s-expressions for those of you familiar with lisp jargon. Unfortunately not all possible binary trees or s-expressions with 0 are meaningful (for example (0)). There must be an even more elegant mapping, but I couldn't think of one so far!

#### 1 comment:

Deniz said...

The official name for the primal notation is "exponent vector" of an integer: "A Tale of Two Sieves" by Pomerance.