21787021 has 4 divisors (see below), whose sum is σ = 21840700. Its totient is φ = 21733344.

The previous prime is 21787009. The next prime is 21787027. The reversal of 21787021 is 12078712.

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 10797796 + 10989225 = 3286^2 + 3315^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21787021 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 (21787027) 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, 26226 + ... + 27043.

It is an arithmetic number, because the mean of its divisors is an integer number (5460175).

Almost surely, 2^21787021 is an apocalyptic number.

It is an amenable number.

21787021 is a deficient number, since it is larger than the sum of its proper divisors (53679).

21787021 is an equidigital number, since it uses as much as digits as its factorization.

21787021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 53678.

The product of its (nonzero) digits is 1568, while the sum is 28.

The square root of 21787021 is about 4667.6569068431. The cubic root of 21787021 is about 279.2967940857.

The spelling of 21787021 in words is "twenty-one million, seven hundred eighty-seven thousand, twenty-one".