21099721 has 4 divisors (see below), whose sum is σ = 21129472. Its totient is φ = 21069972.

The previous prime is 21099719. The next prime is 21099739. The reversal of 21099721 is 12799012.

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 21099721 - 2¹ = 21099719 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21099751) 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, 13785 + ... + 15238.

It is an arithmetic number, because the mean of its divisors is an integer number (5282368).

Almost surely, 2^21099721 is an apocalyptic number.

It is an amenable number.

21099721 is a deficient number, since it is larger than the sum of its proper divisors (29751).

21099721 is an equidigital number, since it uses as much as digits as its factorization.

21099721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 29750.

The product of its (nonzero) digits is 2268, while the sum is 31.

The square root of 21099721 is about 4593.4432618679. The cubic root of 21099721 is about 276.3284308742.

The spelling of 21099721 in words is "twenty-one million, ninety-nine thousand, seven hundred twenty-one".