The way I was introduced to this was as a math trick: Pick a number 1-9, multiply by 9, then 12345679, and you get a string of whatever number you picked.
Also, pick any three-digit number, multiply by 7, 11, and 13 (or 1001), and you get your three-digit number repeated twice.
12345679 * 9 = 111111111
12345679 * 18 = 222222222
12345679 * 27 = 333333333
12345679 * 36 = 444444444
12345679 * 45 = 555555555
12345679 * 54 = 666666666
12345679 * 63 = 777777777
12345679 * 72 = 888888888
12345679 * 81 = 999999999
12345679 * 999999999 = 12345678987654321