**9900**= 2 x 2 x 3 x 3 x 5 x 5 x 11.

**9900**is a pronic number, the product of two consecutive integers:

**9900**= 99 x 100.

**9900**has two distinct digits in base 2 (10011010101100), base 10 (9900), base 19 (1881), and base 21 (1199), each using two digits the same number of times.

**9900**divides 19

^{10}- 1.

Source: Number Gossip