**198 **= 2 x 3 x 3 x 11.

**198 **is the smallest number with ten representations as a sum of four positive squares:

**198 **= 1

^{2} + 2

^{2} + 7

^{2} + 12

^{2} = 1

^{2} + 4

^{2} + 9

^{2} + 10

^{2} = 2

^{2} + 3

^{2} + 4

^{2} + 13

^{2} = 2

^{2} + 3

^{2} + 8

^{2} + 11

^{2} = 2

^{2} + 5

^{2} + 5

^{2} + 12

^{2} = 2

^{2} + 7

^{2} + 8

^{2} + 9

^{2} = 3

^{2} + 3

^{2} + 6

^{2} + 12

^{2} = 3

^{2} + 5

^{2} + 8

^{2} + 10

^{2} = 4

^{2} + 5

^{2} + 6

^{2} + 11

^{2} = 6

^{2} + 7

^{2} + 7

^{2} + 8

^{2}.

**198 **is the first number between a twin prime pair such that if multiplied by its reversal (

**198** x 891 = 176418), the product is also between a twin prime pair.

**198 **can be formed by concatenating 19 and 8, where 19 is the 8th prime number.

**198 **is the (19 x 8 = 152)nd composite number.

**198 **= 11 + 99 + 88 = (100 - 1)(100-98).

**198** x 27 = 5346 = 18 x 297. Each equality uses each of the nine digits 1 to 9 once.

Source:

Prime Curios!