21009857 has 4 divisors (see below), whose sum is σ = 22919856. Its totient is φ = 19099860.

The previous prime is 21009851. The next prime is 21009869. The reversal of 21009857 is 75890012.

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 21009857 - 2¹⁰ = 21008833 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21009851) 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, 954983 + ... + 955004.

It is an arithmetic number, because the mean of its divisors is an integer number (5729964).

Almost surely, 2^21009857 is an apocalyptic number.

It is an amenable number.

21009857 is a deficient number, since it is larger than the sum of its proper divisors (1909999).

21009857 is a wasteful number, since it uses less digits than its factorization.

21009857 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1909998.

The product of its (nonzero) digits is 5040, while the sum is 32.

The square root of 21009857 is about 4583.6510556542. The cubic root of 21009857 is about 275.9355771013.

Adding to 21009857 its reverse (75890012), we get a palindrome (96899869).

The spelling of 21009857 in words is "twenty-one million, nine thousand, eight hundred fifty-seven".