9900

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 19¹⁰ - 1.

Source: Number Gossip