21688021 has 4 divisors (see below), whose sum is σ = 21698892. Its totient is φ = 21677152.

The previous prime is 21688013. The next prime is 21688049. The reversal of 21688021 is 12088612.

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 8328996 + 13359025 = 2886^2 + 3655^2 .

It is a cyclic number.

It is not a de Polignac number, because 21688021 - 2³ = 21688013 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 (21688421) 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, 1486 + ... + 6751.

It is an arithmetic number, because the mean of its divisors is an integer number (5424723).

Almost surely, 2^21688021 is an apocalyptic number.

It is an amenable number.

21688021 is a deficient number, since it is larger than the sum of its proper divisors (10871).

21688021 is an equidigital number, since it uses as much as digits as its factorization.

21688021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10870.

The product of its (nonzero) digits is 1536, while the sum is 28.

The square root of 21688021 is about 4657.0399397042. The cubic root of 21688021 is about 278.8731111097.

The spelling of 21688021 in words is "twenty-one million, six hundred eighty-eight thousand, twenty-one".