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 are 87 distinct three-digit primes in the first thousand digits of π. Note that the initial 3 is ignored. [Ikemoto]