70

This number is a composite.

The prime formed from the reversal of 2⁷⁰ has a digit sum of 70. [Patterson]

Any composite number n has a witness (i.e., does not pass Miller's test) less than 70(ln n)^2 if the Riemann hypothesis is true.

All the primes in the 70's are emirps. [Green]

(There is one curio for this number that has not yet been approved by an editor.)