4939

This number is a composite.

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The smallest number such that if you concatenate the prime factors (4939 = 11 * 449), you form a square (in this case concatenated from 5 square digits: 11449 = 107^2). Also, if you take the sum of digits, you get the square of a prime: 4+9+3+9 = 25 = 5^2. Curiously, the two leftmost digits form another square of a prime. [Post]