21005021 has 4 divisors (see below), whose sum is σ = 21085152. Its totient is φ = 20924892.

The previous prime is 21005009. The next prime is 21005029. The reversal of 21005021 is 12050012.

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 21005021 - 2¹⁴ = 20988637 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21004993 and 21005011.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005029) 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, 39671 + ... + 40196.

It is an arithmetic number, because the mean of its divisors is an integer number (5271288).

Almost surely, 2^21005021 is an apocalyptic number.

It is an amenable number.

21005021 is a deficient number, since it is larger than the sum of its proper divisors (80131).

21005021 is an equidigital number, since it uses as much as digits as its factorization.

21005021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 80130.

The product of its (nonzero) digits is 20, while the sum is 11.

The square root of 21005021 is about 4583.1234982269. The cubic root of 21005021 is about 275.9144040737.

Adding to 21005021 its reverse (12050012), we get a palindrome (33055033).

The spelling of 21005021 in words is "twenty-one million, five thousand, twenty-one".