Create unique number from 2 numbers

The sort of function you are looking for is often called a pairing function.

A standard example is the Cantor pairing function $\mathbb{N}\times\mathbb{N}\rightarrow\mathbb{N}$, given by: $\pi(a,b) = \frac{1}{2}(a+b)(a+b+1) + b$.

You can find more information here: http://en.wikipedia.org/wiki/pairing_function

if the numbers are $a$ and $b$, take $2^a3^b$. This method works for any number of numbers (just take different primes as the bases), and all the numbers are distinct.

Google pairing function. As I mentioned in the similar question, there are also other pairing functions besides the well-known one due to Cantor. For example, see this "elegant" pairing function, which has the useful property that it orders many expressions by depth.

For positive integers as arguments and where argument order doesn't matter:

1. Here's an unordered pairing function:

   $<x, y> = x * y + trunc(\frac{(|x - y| - 1)^2}{4}) = <y, x>$

2. For x ≠ y, here's a unique unordered pairing function:

   <x, y> = if x < y:
              x * (y - 1) + trunc((y - x - 2)^2 / 4)
            if x > y:
              (x - 1) * y + trunc((x - y - 2)^2 / 4)
          = <y, x>