198 is the smallest number with ten representations as a sum of four positive squares: 198 = 12 + 22 + 72 + 122 = 12 + 42 + 92 + 102 = 22 + 32 + 42 + 132 = 22 + 32 + 82 + 112 = 22 + 52 + 52 + 122 = 22 + 72 + 82 + 92 = 32 + 32 + 62 + 122 = 32 + 52 + 82 + 102 = 42 + 52 + 62 + 112 = 62 + 72 + 72 + 82.
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!
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