21665801 has 4 divisors (see below), whose sum is σ = 21715152. Its totient is φ = 21616452.

The previous prime is 21665789. The next prime is 21665827. The reversal of 21665801 is 10856612.

It is a happy number.

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 21665801 - 2⁶ = 21665737 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21665101) 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, 24011 + ... + 24896.

It is an arithmetic number, because the mean of its divisors is an integer number (5428788).

Almost surely, 2^21665801 is an apocalyptic number.

It is an amenable number.

21665801 is a deficient number, since it is larger than the sum of its proper divisors (49351).

21665801 is an equidigital number, since it uses as much as digits as its factorization.

21665801 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 49350.

The product of its (nonzero) digits is 2880, while the sum is 29.

The square root of 21665801 is about 4654.6536928111. The cubic root of 21665801 is about 278.7778407311. Note that the first 3 decimals are identical.

The spelling of 21665801 in words is "twenty-one million, six hundred sixty-five thousand, eight hundred one".