21205765 has 8 divisors (see below), whose sum is σ = 29082240. Its totient is φ = 14541072.

The previous prime is 21205759. The next prime is 21205781. The reversal of 21205765 is 56750212.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21205765 - 2⁷ = 21205637 is a prime.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 302905 + ... + 302974.

It is an arithmetic number, because the mean of its divisors is an integer number (3635280).

Almost surely, 2^21205765 is an apocalyptic number.

It is an amenable number.

21205765 is a deficient number, since it is larger than the sum of its proper divisors (7876475).

21205765 is an equidigital number, since it uses as much as digits as its factorization.

21205765 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 605891.

The product of its (nonzero) digits is 4200, while the sum is 28.

The square root of 21205765 is about 4604.9717697289. The cubic root of 21205765 is about 276.7905857683.

Adding to 21205765 its reverse (56750212), we get a palindrome (77955977).

The spelling of 21205765 in words is "twenty-one million, two hundred five thousand, seven hundred sixty-five".