21784721 has 4 divisors (see below), whose sum is σ = 24896832. Its totient is φ = 18672612.

The previous prime is 21784699. The next prime is 21784727. The reversal of 21784721 is 12748712.

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 not a de Polignac number, because 21784721 - 2¹⁴ = 21768337 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21784727) 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, 1556045 + ... + 1556058.

It is an arithmetic number, because the mean of its divisors is an integer number (6224208).

Almost surely, 2^21784721 is an apocalyptic number.

It is an amenable number.

21784721 is a deficient number, since it is larger than the sum of its proper divisors (3112111).

21784721 is an equidigital number, since it uses as much as digits as its factorization.

21784721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3112110.

The product of its digits is 6272, while the sum is 32.

The square root of 21784721 is about 4667.4105240486. The cubic root of 21784721 is about 279.2869655241.

The spelling of 21784721 in words is "twenty-one million, seven hundred eighty-four thousand, seven hundred twenty-one".