21435721 has 4 divisors (see below), whose sum is σ = 21445072. Its totient is φ = 21426372.

The previous prime is 21435719. The next prime is 21435731. The reversal of 21435721 is 12753412.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21435721 - 2¹ = 21435719 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21435731) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1366 + ... + 6688.

It is an arithmetic number, because the mean of its divisors is an integer number (5361268).

Almost surely, 2^21435721 is an apocalyptic number.

It is an amenable number.

21435721 is a deficient number, since it is larger than the sum of its proper divisors (9351).

21435721 is an equidigital number, since it uses as much as digits as its factorization.

21435721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9350.

The product of its digits is 1680, while the sum is 25.

The square root of 21435721 is about 4629.8726764351. The cubic root of 21435721 is about 277.7874995649.

The spelling of 21435721 in words is "twenty-one million, four hundred thirty-five thousand, seven hundred twenty-one".