80

This number is a composite.

There are 80 four-digit primes which are concatenations of two-digit primes. [De Geest]

(10⁸⁰ - 80)/80 is prime. [Luhn]

80^(80+1) + (80+1)^80 is prime.

The largest natural number n such that all prime factors of n and n+1 are smaller or equal to the prime digit 5. [Capelle]

The only two-digit integer n that yields a prime in the form n^(n+1)+(n+1)^n. [Silva]

The smallest integer n such that both n and n+1 are products of at least 4 primes. [Post]

Theoretically, the fewest prime factors in the numbers spanning a non-trivial 8-tuple. [Merickel]