21000021 has 16 divisors (see below), whose sum is σ = 32320128. Its totient is φ = 11880000.

The previous prime is 20999999. The next prime is 21000037. The reversal of 21000021 is 12000012.

It is a happy number.

It is not a de Polignac number, because 21000021 - 2⁷ = 20999893 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 20999973 and 21000009.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21000041) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2830 + ... + 7071.

It is an arithmetic number, because the mean of its divisors is an integer number (2020008).

Almost surely, 2^21000021 is an apocalyptic number.

21000021 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21000021 is a deficient number, since it is larger than the sum of its proper divisors (11320107).

21000021 is a wasteful number, since it uses less digits than its factorization.

21000021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10012.

The product of its (nonzero) digits is 4, while the sum is 6.

The square root of 21000021 is about 4582.5779862431. The cubic root of 21000021 is about 275.8925096022.

Adding to 21000021 its reverse (12000012), we get a palindrome (33000033).

Subtracting from 21000021 its reverse (12000012), we obtain a palindrome (9000009).

It can be divided in two parts, 210000 and 21, that multiplied together give a square (4410000 = 2100²).

The spelling of 21000021 in words is "twenty-one million, twenty-one", and thus it is an aban number and an uban number.