21550721 has 4 divisors (see below), whose sum is σ = 21663744. Its totient is φ = 21437700.

The previous prime is 21550691. The next prime is 21550777. The reversal of 21550721 is 12705512.

It is a happy number.

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 21550721 - 2⁶ = 21550657 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21558721) 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, 56225 + ... + 56606.

It is an arithmetic number, because the mean of its divisors is an integer number (5415936).

Almost surely, 2^21550721 is an apocalyptic number.

It is an amenable number.

21550721 is a deficient number, since it is larger than the sum of its proper divisors (113023).

21550721 is a wasteful number, since it uses less digits than its factorization.

21550721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 113022.

The product of its (nonzero) digits is 700, while the sum is 23.

The square root of 21550721 is about 4642.2754119074. The cubic root of 21550721 is about 278.2833790692.

The spelling of 21550721 in words is "twenty-one million, five hundred fifty thousand, seven hundred twenty-one".