21017851 has 4 divisors (see below), whose sum is σ = 21105304. Its totient is φ = 20930400.

The previous prime is 21017809. The next prime is 21017861. The reversal of 21017851 is 15871012.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21017851 - 2¹¹ = 21015803 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21017861) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 43365 + ... + 43846.

It is an arithmetic number, because the mean of its divisors is an integer number (5276326).

Almost surely, 2^21017851 is an apocalyptic number.

21017851 is a deficient number, since it is larger than the sum of its proper divisors (87453).

21017851 is an equidigital number, since it uses as much as digits as its factorization.

21017851 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 87452.

The product of its (nonzero) digits is 560, while the sum is 25.

The square root of 21017851 is about 4584.5229850007. The cubic root of 21017851 is about 275.9705693956.

Adding to 21017851 its reverse (15871012), we get a palindrome (36888863).

The spelling of 21017851 in words is "twenty-one million, seventeen thousand, eight hundred fifty-one".