209872021 has 4 divisors (see below), whose sum is σ = 222217452. Its totient is φ = 197526592.

The previous prime is 209872001. The next prime is 209872037. The reversal of 209872021 is 120278902.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 63123025 + 146748996 = 7945^2 + 12114^2 .

It is a cyclic number.

It is not a de Polignac number, because 209872021 - 2¹¹ = 209869973 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (209872001) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6172690 + ... + 6172723.

It is an arithmetic number, because the mean of its divisors is an integer number (55554363).

Almost surely, 2^209872021 is an apocalyptic number.

It is an amenable number.

209872021 is a deficient number, since it is larger than the sum of its proper divisors (12345431).

209872021 is a wasteful number, since it uses less digits than its factorization.

209872021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12345430.

The product of its (nonzero) digits is 4032, while the sum is 31.

The square root of 209872021 is about 14486.9603782160. The cubic root of 209872021 is about 594.2714251543.

The spelling of 209872021 in words is "two hundred nine million, eight hundred seventy-two thousand, twenty-one".