Originally posted by WulebgrAdding up the divisors of 1 through 9 gives 69.
What is so special about the number 69?
Because 69 has an odd number of 1s in its binary representation, it is sometimes called an "odious number" (see Thue-Morse sequence).
It is the largest number whose factorial is less than 10100 (69! ≈ 1.71x1098). This means it is often the largest factorial that can be calculated by a pocket calculator. (See 449.)
It also has the interesting property of being the only number whose square and cube contain once and only once all numerals from zero to nine when written in decimal notation:
* 692 = 4761
* 693 = 328509
Also, interesting is that 6932 = 6969...
Another interesting property is that 69 is equal to 105 octal, while 105 is equal to 69 hexadecimal. This same property can be applied to all numbers from 64 to 69.
69 is a semiprime.
How can you not think thats special?