21085561 has 4 divisors (see below), whose sum is σ = 24097792. Its totient is φ = 18073332.

The previous prime is 21085543. The next prime is 21085567. The reversal of 21085561 is 16558012.

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 21085561 - 2⁵ = 21085529 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21085567) 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, 1506105 + ... + 1506118.

It is an arithmetic number, because the mean of its divisors is an integer number (6024448).

Almost surely, 2^21085561 is an apocalyptic number.

It is an amenable number.

21085561 is a deficient number, since it is larger than the sum of its proper divisors (3012231).

21085561 is an equidigital number, since it uses as much as digits as its factorization.

21085561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3012230.

The product of its (nonzero) digits is 2400, while the sum is 28.

The square root of 21085561 is about 4591.9016757766. The cubic root of 21085561 is about 276.2666024706.

The spelling of 21085561 in words is "twenty-one million, eighty-five thousand, five hundred sixty-one".