21801721 has 4 divisors (see below), whose sum is σ = 22949200. Its totient is φ = 20654244.

The previous prime is 21801719. The next prime is 21801727. The reversal of 21801721 is 12710812.

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 21801721 - 2¹ = 21801719 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21801692 and 21801701.

It is not an unprimeable number, because it can be changed into a prime (21801727) 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, 573711 + ... + 573748.

It is an arithmetic number, because the mean of its divisors is an integer number (5737300).

Almost surely, 2^21801721 is an apocalyptic number.

It is an amenable number.

21801721 is a deficient number, since it is larger than the sum of its proper divisors (1147479).

21801721 is a wasteful number, since it uses less digits than its factorization.

21801721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1147478.

The product of its (nonzero) digits is 224, while the sum is 22.

The square root of 21801721 is about 4669.2313071854. The cubic root of 21801721 is about 279.3595950814.

Subtracting from 21801721 its reverse (12710812), we obtain a palindrome (9090909).

It can be divided in two parts, 21801 and 721, that added together give a palindrome (22522).

The spelling of 21801721 in words is "twenty-one million, eight hundred one thousand, seven hundred twenty-one".