21587721 has 4 divisors (see below), whose sum is σ = 28783632. Its totient is φ = 14391812.

The previous prime is 21587707. The next prime is 21587723. The reversal of 21587721 is 12778512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21587721 - 2¹¹ = 21585673 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21587723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3597951 + ... + 3597956.

It is an arithmetic number, because the mean of its divisors is an integer number (7195908).

Almost surely, 2^21587721 is an apocalyptic number.

It is an amenable number.

21587721 is a deficient number, since it is larger than the sum of its proper divisors (7195911).

21587721 is an equidigital number, since it uses as much as digits as its factorization.

21587721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7195910.

The product of its digits is 7840, while the sum is 33.

The square root of 21587721 is about 4646.2588175865. The cubic root of 21587721 is about 278.4425477274.

The spelling of 21587721 in words is "twenty-one million, five hundred eighty-seven thousand, seven hundred twenty-one".