21856397 has 4 divisors (see below), whose sum is σ = 21867420. Its totient is φ = 21845376.

The previous prime is 21856391. The next prime is 21856399. The reversal of 21856397 is 79365812.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 8099716 + 13756681 = 2846^2 + 3709^2 .

It is a cyclic number.

It is not a de Polignac number, because 21856397 - 2¹⁸ = 21594253 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 (21856391) 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, 1622 + ... + 6807.

It is an arithmetic number, because the mean of its divisors is an integer number (5466855).

Almost surely, 2^21856397 is an apocalyptic number.

It is an amenable number.

21856397 is a deficient number, since it is larger than the sum of its proper divisors (11023).

21856397 is an equidigital number, since it uses as much as digits as its factorization.

21856397 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 11022.

The product of its digits is 90720, while the sum is 41.

The square root of 21856397 is about 4675.0825661158. The cubic root of 21856397 is about 279.5929331487.

The spelling of 21856397 in words is "twenty-one million, eight hundred fifty-six thousand, three hundred ninety-seven".