21397821 has 8 divisors (see below), whose sum is σ = 28554240. Its totient is φ = 14253312.

The previous prime is 21397813. The next prime is 21397823. The reversal of 21397821 is 12879312.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21397821 - 2³ = 21397813 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21397823) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2845 + ... + 7133.

It is an arithmetic number, because the mean of its divisors is an integer number (3569280).

Almost surely, 2^21397821 is an apocalyptic number.

It is an amenable number.

21397821 is a deficient number, since it is larger than the sum of its proper divisors (7156419).

21397821 is a wasteful number, since it uses less digits than its factorization.

21397821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5955.

The product of its digits is 6048, while the sum is 33.

The square root of 21397821 is about 4625.7778805299. Note that the first 3 decimals coincide. The cubic root of 21397821 is about 277.6236864421.

The spelling of 21397821 in words is "twenty-one million, three hundred ninety-seven thousand, eight hundred twenty-one".