21390821 has 4 divisors (see below), whose sum is σ = 21412044. Its totient is φ = 21369600.

The previous prime is 21390797. The next prime is 21390833. The reversal of 21390821 is 12809312.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 14025025 + 7365796 = 3745^2 + 2714^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21390821 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 (21390871) 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, 9020 + ... + 11141.

It is an arithmetic number, because the mean of its divisors is an integer number (5353011).

Almost surely, 2^21390821 is an apocalyptic number.

It is an amenable number.

21390821 is a deficient number, since it is larger than the sum of its proper divisors (21223).

21390821 is a wasteful number, since it uses less digits than its factorization.

21390821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 21222.

The product of its (nonzero) digits is 864, while the sum is 26.

The square root of 21390821 is about 4625.0211891407. The cubic root of 21390821 is about 277.5934095624.

The spelling of 21390821 in words is "twenty-one million, three hundred ninety thousand, eight hundred twenty-one".