21081689 has 4 divisors (see below), whose sum is σ = 21299124. Its totient is φ = 20864256.

The previous prime is 21081679. The next prime is 21081691. The reversal of 21081689 is 98618012.

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 4000000 + 17081689 = 2000^2 + 4133^2 .

It is a cyclic number.

It is not a de Polignac number, because 21081689 - 2⁴ = 21081673 is a prime.

It is a super-2 number, since 2×21081689² = 888875222185442, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21081679) 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, 108572 + ... + 108765.

It is an arithmetic number, because the mean of its divisors is an integer number (5324781).

Almost surely, 2^21081689 is an apocalyptic number.

It is an amenable number.

21081689 is a deficient number, since it is larger than the sum of its proper divisors (217435).

21081689 is an equidigital number, since it uses as much as digits as its factorization.

21081689 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 217434.

The product of its (nonzero) digits is 6912, while the sum is 35.

The square root of 21081689 is about 4591.4800446044. The cubic root of 21081689 is about 276.2496909014.

The spelling of 21081689 in words is "twenty-one million, eighty-one thousand, six hundred eighty-nine".