21809851 has 12 divisors (see below), whose sum is σ = 25732080. Its totient is φ = 18427920.

The previous prime is 21809789. The next prime is 21809863. The reversal of 21809851 is 15890812.

It is not a de Polignac number, because 21809851 - 2¹¹ = 21807803 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21809251) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 345 + ... + 6613.

It is an arithmetic number, because the mean of its divisors is an integer number (2144340).

Almost surely, 2^21809851 is an apocalyptic number.

21809851 is a deficient number, since it is larger than the sum of its proper divisors (3922229).

21809851 is an equidigital number, since it uses as much as digits as its factorization.

21809851 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6354 (or 6347 counting only the distinct ones).

The product of its (nonzero) digits is 5760, while the sum is 34.

The square root of 21809851 is about 4670.1018190185. The cubic root of 21809851 is about 279.3943157540.

The spelling of 21809851 in words is "twenty-one million, eight hundred nine thousand, eight hundred fifty-one".