21187861 has 4 divisors (see below), whose sum is σ = 21235500. Its totient is φ = 21140224.

The previous prime is 21187847. The next prime is 21187871. The reversal of 21187861 is 16878112.

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 18232900 + 2954961 = 4270^2 + 1719^2 .

It is a cyclic number.

It is not a de Polignac number, because 21187861 - 2⁹ = 21187349 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21187811) 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, 23146 + ... + 24043.

It is an arithmetic number, because the mean of its divisors is an integer number (5308875).

Almost surely, 2^21187861 is an apocalyptic number.

It is an amenable number.

21187861 is a deficient number, since it is larger than the sum of its proper divisors (47639).

21187861 is an equidigital number, since it uses as much as digits as its factorization.

21187861 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 47638.

The product of its digits is 5376, while the sum is 34.

The square root of 21187861 is about 4603.0273733707. The cubic root of 21187861 is about 276.7126658569.

The spelling of 21187861 in words is "twenty-one million, one hundred eighty-seven thousand, eight hundred sixty-one".