21784561 has 4 divisors (see below), whose sum is σ = 21794112. Its totient is φ = 21775012.

The previous prime is 21784559. The next prime is 21784673. The reversal of 21784561 is 16548712.

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 21784561 - 2¹ = 21784559 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21784261) 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, 876 + ... + 6658.

It is an arithmetic number, because the mean of its divisors is an integer number (5448528).

Almost surely, 2^21784561 is an apocalyptic number.

It is an amenable number.

21784561 is a deficient number, since it is larger than the sum of its proper divisors (9551).

21784561 is an equidigital number, since it uses as much as digits as its factorization.

21784561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9550.

The product of its digits is 13440, while the sum is 34.

The square root of 21784561 is about 4667.3933838921. The cubic root of 21784561 is about 279.2862817723.

The spelling of 21784561 in words is "twenty-one million, seven hundred eighty-four thousand, five hundred sixty-one".