21138857 has 4 divisors (see below), whose sum is σ = 21149700. Its totient is φ = 21128016.

The previous prime is 21138853. The next prime is 21138881. The reversal of 21138857 is 75883112.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 12752041 + 8386816 = 3571^2 + 2896^2 .

It is a cyclic number.

It is not a de Polignac number, because 21138857 - 2² = 21138853 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21138851) 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, 1598 + ... + 6695.

It is an arithmetic number, because the mean of its divisors is an integer number (5287425).

Almost surely, 2^21138857 is an apocalyptic number.

It is an amenable number.

21138857 is a deficient number, since it is larger than the sum of its proper divisors (10843).

21138857 is an equidigital number, since it uses as much as digits as its factorization.

21138857 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10842.

The product of its digits is 13440, while the sum is 35.

The square root of 21138857 is about 4597.7012734626. The cubic root of 21138857 is about 276.4991710487.

The spelling of 21138857 in words is "twenty-one million, one hundred thirty-eight thousand, eight hundred fifty-seven".