43

This number is a prime.

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43 = 4*prime(4) + 3*prime(3). Note that 43 is the earliest number with this property. There are only two such numbers and both are primes. [Firoozbakht]

4^4 - 3^3 is a prime with length π(π(π(43))). Note there is only one prime (43) and one composite number with this property. [Firoozbakht]