21205771 has 4 divisors (see below), whose sum is σ = 21265200. Its totient is φ = 21146344.

The previous prime is 21205759. The next prime is 21205781. The reversal of 21205771 is 17750212.

It is a happy number.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21205771 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21205781) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 29176 + ... + 29893.

It is an arithmetic number, because the mean of its divisors is an integer number (5316300).

Almost surely, 2^21205771 is an apocalyptic number.

21205771 is a deficient number, since it is larger than the sum of its proper divisors (59429).

21205771 is an equidigital number, since it uses as much as digits as its factorization.

21205771 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 59428.

The product of its (nonzero) digits is 980, while the sum is 25.

The square root of 21205771 is about 4604.9724211986. The cubic root of 21205771 is about 276.7906118735.

Adding to 21205771 its reverse (17750212), we get a palindrome (38955983).

The spelling of 21205771 in words is "twenty-one million, two hundred five thousand, seven hundred seventy-one".