21857421 has 16 divisors (see below), whose sum is σ = 29911680. Its totient is φ = 14192640.

The previous prime is 21857411. The next prime is 21857431. The reversal of 21857421 is 12475812.

It is an interprime number because it is at equal distance from previous prime (21857411) and next prime (21857431).

It is not a de Polignac number, because 21857421 - 2⁵ = 21857389 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21857411) 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 15 ways as a sum of consecutive naturals, for example, 18381 + ... + 19533.

It is an arithmetic number, because the mean of its divisors is an integer number (1869480).

Almost surely, 2^21857421 is an apocalyptic number.

It is an amenable number.

21857421 is a deficient number, since it is larger than the sum of its proper divisors (8054259).

21857421 is a wasteful number, since it uses less digits than its factorization.

21857421 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1316.

The product of its digits is 4480, while the sum is 30.

The square root of 21857421 is about 4675.1920816155. The cubic root of 21857421 is about 279.5972995087.

The spelling of 21857421 in words is "twenty-one million, eight hundred fifty-seven thousand, four hundred twenty-one".