209921 has 4 divisors (see below), whose sum is σ = 219072. Its totient is φ = 200772.

The previous prime is 209917. The next prime is 209927. The reversal of 209921 is 129902.

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: 129902 = 2 ⋅64951.

It is a cyclic number.

It is not a de Polignac number, because 209921 - 2² = 209917 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (23), and also a Moran number because the ratio is a prime number: 9127 = 209921 / (2 + 0 + 9 + 9 + 2 + 1).

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (209927) 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, 4541 + ... + 4586.

It is an arithmetic number, because the mean of its divisors is an integer number (54768).

It is a Proth number, since it is equal to 205 ⋅ 2¹⁰ + 1 and 205 < 2¹⁰.

2²⁰⁹⁹²¹ is an apocalyptic number.

It is an amenable number.

209921 is a deficient number, since it is larger than the sum of its proper divisors (9151).

209921 is an equidigital number, since it uses as much as digits as its factorization.

209921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9150.

The product of its (nonzero) digits is 324, while the sum is 23.

The square root of 209921 is about 458.1713653209. The cubic root of 209921 is about 59.4317651034.

The spelling of 209921 in words is "two hundred nine thousand, nine hundred twenty-one".