21297721 has 4 divisors (see below), whose sum is σ = 21750912. Its totient is φ = 20844532.

The previous prime is 21297691. The next prime is 21297733. The reversal of 21297721 is 12779212.

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 21297721 - 2¹⁹ = 20773433 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21297751) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 226525 + ... + 226618.

It is an arithmetic number, because the mean of its divisors is an integer number (5437728).

Almost surely, 2^21297721 is an apocalyptic number.

It is an amenable number.

21297721 is a deficient number, since it is larger than the sum of its proper divisors (453191).

21297721 is an equidigital number, since it uses as much as digits as its factorization.

21297721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 453190.

The product of its digits is 3528, while the sum is 31.

The square root of 21297721 is about 4614.9453951266. The cubic root of 21297721 is about 277.1900974566.

The spelling of 21297721 in words is "twenty-one million, two hundred ninety-seven thousand, seven hundred twenty-one".