21689561 has 4 divisors (see below), whose sum is σ = 22098852. Its totient is φ = 21280272.

The previous prime is 21689557. The next prime is 21689573. The reversal of 21689561 is 16598612.

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 8323225 + 13366336 = 2885^2 + 3656^2 .

It is a cyclic number.

It is not a de Polignac number, because 21689561 - 2² = 21689557 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21689531) 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, 204566 + ... + 204671.

It is an arithmetic number, because the mean of its divisors is an integer number (5524713).

Almost surely, 2^21689561 is an apocalyptic number.

It is an amenable number.

21689561 is a deficient number, since it is larger than the sum of its proper divisors (409291).

21689561 is an equidigital number, since it uses as much as digits as its factorization.

21689561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 409290.

The product of its digits is 25920, while the sum is 38.

The square root of 21689561 is about 4657.2052778464. The cubic root of 21689561 is about 278.8797115956.

The spelling of 21689561 in words is "twenty-one million, six hundred eighty-nine thousand, five hundred sixty-one".