9931 is a prime number.
9929 and 9931 form a twin prime pair. 9931 is the larger of the greatest twin prime pair with four digits (A114429).
9931 and its reversal, 1399, are both primes (A101782).
9931 is a prime with 10 as the smallest positive primitive root (A061323).
9931 is a prime p such that p - 2 and p3 - 2 are also prime (A240124).
9931 is a prime that is the sum of 8 but no fewer squared primes (A183216).