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# 87

This number is a composite.

87 = 3 * 29, and curiously 87^{2} + 3^{2} + 29^{2} and 87^{2} - 3^{2} - 29^{2} are primes. [Luhn]

87 = 2^{2} + 3^{2} + 5^{2} + 7^{2}. [Sladcik]

87 is the largest number that yields a prime when any of the one-digit primes 2, 5 or 7 is inserted between any two digits. The only other such number is 27 (and trivially, the 1 digit numbers). [Hasler]

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