21551721 has 4 divisors (see below), whose sum is σ = 28735632. Its totient is φ = 14367812.

The previous prime is 21551701. The next prime is 21551737. The reversal of 21551721 is 12715512.

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 21551721 - 2⁶ = 21551657 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21551691 and 21551700.

It is not an unprimeable number, because it can be changed into a prime (21551701) 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, 3591951 + ... + 3591956.

It is an arithmetic number, because the mean of its divisors is an integer number (7183908).

Almost surely, 2^21551721 is an apocalyptic number.

It is an amenable number.

21551721 is a deficient number, since it is larger than the sum of its proper divisors (7183911).

21551721 is an equidigital number, since it uses as much as digits as its factorization.

21551721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7183910.

The product of its digits is 700, while the sum is 24.

The square root of 21551721 is about 4642.3831164608. The cubic root of 21551721 is about 278.2876833193.

The spelling of 21551721 in words is "twenty-one million, five hundred fifty-one thousand, seven hundred twenty-one".