This number is a composite.

+ 198 is the first number between a twin prime pair such that if multiplied by its reversal (198 * 891) does the same! [Luhn]

+ 198 can be formed by concatenating 19 and 8. Note that 19 is the 8th prime number. Curiously, 198 is the (19*8)nd composite number! [Fazely]

+ The difference between any odd-digit emirp pairs is divisible by 198. [Green]

(There is one curio for this number that has not yet been approved by an editor.)

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