12

This number is a composite.

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+ The smallest composite number ending in a prime digit. [Silva]

+ The concatenation of the difference and the sum of number 12 and the 12th prime is prime (2549). Note that the concatenation of 12 and the 12th prime is an emirp. [Silva]

+ 12 is the number of prime digits appearing in the first 12 primes and 21 is the number of prime digits appearing in the first 21 primes. [Silva]

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

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