9973

This number is a prime.

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The largest 4-digit prime whose sum of digits is a perfect number. [Loungrides]

The only 4-digit prime formed from the concatenation of two primes summed to a thousand, (997, 3). [Loungrides]

The largest 4-digit prime 9973 can be read as “99 is the 73rd composite number.” [Loungrides]