21645021 has 4 divisors (see below), whose sum is σ = 28860032. Its totient is φ = 14430012.

The previous prime is 21644969. The next prime is 21645023. The reversal of 21645021 is 12054612.

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 not a de Polignac number, because 21645021 - 2⁹ = 21644509 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21645023) 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, 3607501 + ... + 3607506.

It is an arithmetic number, because the mean of its divisors is an integer number (7215008).

Almost surely, 2^21645021 is an apocalyptic number.

It is an amenable number.

21645021 is a deficient number, since it is larger than the sum of its proper divisors (7215011).

21645021 is an equidigital number, since it uses as much as digits as its factorization.

21645021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7215010.

The product of its (nonzero) digits is 480, while the sum is 21.

The square root of 21645021 is about 4652.4209826713. The cubic root of 21645021 is about 278.6886855295.

Adding to 21645021 its reverse (12054612), we get a palindrome (33699633).

The spelling of 21645021 in words is "twenty-one million, six hundred forty-five thousand, twenty-one".