21158561 has 4 divisors (see below), whose sum is σ = 21730452. Its totient is φ = 20586672.

The previous prime is 21158549. The next prime is 21158563. The reversal of 21158561 is 16585112.

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 11971600 + 9186961 = 3460^2 + 3031^2 .

It is a cyclic number.

It is not a de Polignac number, because 21158561 - 2²² = 16964257 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21158563) 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, 285890 + ... + 285963.

It is an arithmetic number, because the mean of its divisors is an integer number (5432613).

Almost surely, 2^21158561 is an apocalyptic number.

It is an amenable number.

21158561 is a deficient number, since it is larger than the sum of its proper divisors (571891).

21158561 is an equidigital number, since it uses as much as digits as its factorization.

21158561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 571890.

The product of its digits is 2400, while the sum is 29.

The square root of 21158561 is about 4599.8435842972. The cubic root of 21158561 is about 276.5850547173.

The spelling of 21158561 in words is "twenty-one million, one hundred fifty-eight thousand, five hundred sixty-one".