Here's a little math problem for you.

Say I want to create a page containing every 7-digit phone number (ignore the fact that some combinations of numbers aren't allowed, like "555" in the prefix). What's the minimum number of characters that can be used to create the page? It's not 7*10^7, because the string "12345678" contains two 7-digit numbers: "1234567" and "2345678". The real question then is, what algorithm can be used to generate a single long sequence of numbers that contains every 7-digit combination with as few repeats as possible? How long will that sequence be?