87
This number is a composite.
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 is one curio for this number that has not yet been approved by an editor.)
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