87

+ 87 = 3 * 29, and curiously 872 + 32 + 292 and 872 - 32 - 292 are primes. [Luhn]

+ 87 = 22 + 32 + 52 + 72. [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]

(There are 3 curios for this number that have not yet been approved by an editor.)

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