1681

This number is a composite.

+ First square K with prime triplets K-18, K-14, K-12, K+12, K+16, K+18 as nearest primes (next one, if any, is over 10^7). [Wood]

+ The smallest composite (semiprime) generated from Euler's famous prime polynomial x^2+x+41. [Gupta]

+ The smallest square whose representations in septenary, octal and nonary (4621, 3221, 2267) are all prime when read in decimal. Note that 1681 = 2^4||3^4 i.e., concatenation of fourth-powers of first two primes. [Bajpai]

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