21020021 has 4 divisors (see below), whose sum is σ = 22930944. Its totient is φ = 19109100.

The previous prime is 21020017. The next prime is 21020029. The reversal of 21020021 is 12002012.

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.

It is a cyclic number.

It is not a de Polignac number, because 21020021 - 2² = 21020017 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21019990 and 21020008.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21020029) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 955445 + ... + 955466.

It is an arithmetic number, because the mean of its divisors is an integer number (5732736).

Almost surely, 2^21020021 is an apocalyptic number.

It is an amenable number.

21020021 is a deficient number, since it is larger than the sum of its proper divisors (1910923).

21020021 is a wasteful number, since it uses less digits than its factorization.

21020021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1910922.

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

The square root of 21020021 is about 4584.7596447360. The cubic root of 21020021 is about 275.9800666483.

Adding to 21020021 its reverse (12002012), we get a palindrome (33022033).

Subtracting from 21020021 its reverse (12002012), we obtain a square (9018009 = 3003²).

The spelling of 21020021 in words is "twenty-one million, twenty thousand, twenty-one".