21688591 has 4 divisors (see below), whose sum is σ = 21719152. Its totient is φ = 21658032.

The previous prime is 21688571. The next prime is 21688603. The reversal of 21688591 is 19588612.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21688591 - 2⁵ = 21688559 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 (21688531) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 14190 + ... + 15643.

It is an arithmetic number, because the mean of its divisors is an integer number (5429788).

Almost surely, 2^21688591 is an apocalyptic number.

21688591 is a deficient number, since it is larger than the sum of its proper divisors (30561).

21688591 is an equidigital number, since it uses as much as digits as its factorization.

21688591 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 30560.

The product of its digits is 34560, while the sum is 40.

The square root of 21688591 is about 4657.1011369735. The cubic root of 21688591 is about 278.8755541831.

The spelling of 21688591 in words is "twenty-one million, six hundred eighty-eight thousand, five hundred ninety-one".