21210021 has 12 divisors (see below), whose sum is σ = 35013472. Its totient is φ = 12119976.

The previous prime is 21210011. The next prime is 21210037. The reversal of 21210021 is 12001212.

It is not a de Polignac number, because 21210021 - 2⁶ = 21209957 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21209985 and 21210012.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21210011) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 168271 + ... + 168396.

Almost surely, 2^21210021 is an apocalyptic number.

21210021 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21210021 is a deficient number, since it is larger than the sum of its proper divisors (13803451).

21210021 is a wasteful number, since it uses less digits than its factorization.

21210021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 336680 (or 336677 counting only the distinct ones).

The product of its (nonzero) digits is 8, while the sum is 9.

The square root of 21210021 is about 4605.4338557838. The cubic root of 21210021 is about 276.8091018327.

Adding to 21210021 its reverse (12001212), we get a palindrome (33211233).

The spelling of 21210021 in words is "twenty-one million, two hundred ten thousand, twenty-one".