21704561 has 4 divisors (see below), whose sum is σ = 21778932. Its totient is φ = 21630192.

The previous prime is 21704549. The next prime is 21704567. The reversal of 21704561 is 16540712.

21704561 = T₂₂₉ + T₂₃₀ + ... + T₅₂₁.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 13690000 + 8014561 = 3700^2 + 2831^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21704561 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21704561.

It is not an unprimeable number, because it can be changed into a prime (21704567) 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, 36746 + ... + 37331.

It is an arithmetic number, because the mean of its divisors is an integer number (5444733).

Almost surely, 2^21704561 is an apocalyptic number.

It is an amenable number.

21704561 is a deficient number, since it is larger than the sum of its proper divisors (74371).

21704561 is an equidigital number, since it uses as much as digits as its factorization.

21704561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 74370.

The product of its (nonzero) digits is 1680, while the sum is 26.

The square root of 21704561 is about 4658.8154073756. The cubic root of 21704561 is about 278.9439857059.

The spelling of 21704561 in words is "twenty-one million, seven hundred four thousand, five hundred sixty-one".