9921 has 4 divisors (see below), whose sum is σ = 13232. Its totient is φ = 6612.

The previous prime is 9907. The next prime is 9923. The reversal of 9921 is 1299.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 1299 = 3 ⋅433.

It is not a de Polignac number, because 9921 - 2⁶ = 9857 is a prime.

It is a D-number.

It is a Duffinian number.

It is a Curzon number.

It is a plaindrome in base 14.

It is a nialpdrome in base 10.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 9921.

It is not an unprimeable number, because it can be changed into a prime (9923) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1651 + ... + 1656.

It is an arithmetic number, because the mean of its divisors is an integer number (3308).

2⁹⁹²¹ is an apocalyptic number.

It is an amenable number.

9921 is a deficient number, since it is larger than the sum of its proper divisors (3311).

9921 is a wasteful number, since it uses less digits than its factorization.

9921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3310.

The product of its digits is 162, while the sum is 21.

The square root of 9921 is about 99.6042167782. The cubic root of 9921 is about 21.4874633962.

It can be divided in two parts, 99 and 21, that added together give a triangular number (120 = T₁₅).

The spelling of 9921 in words is "nine thousand, nine hundred twenty-one".