21205717 has 4 divisors (see below), whose sum is σ = 22836940. Its totient is φ = 19574496.

The previous prime is 21205703. The next prime is 21205727. The reversal of 21205717 is 71750212.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 12859396 + 8346321 = 3586^2 + 2889^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21205717 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21205691 and 21205700.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21205727) 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, 815592 + ... + 815617.

It is an arithmetic number, because the mean of its divisors is an integer number (5709235).

Almost surely, 2^21205717 is an apocalyptic number.

It is an amenable number.

21205717 is a deficient number, since it is larger than the sum of its proper divisors (1631223).

21205717 is a wasteful number, since it uses less digits than its factorization.

21205717 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1631222.

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

The square root of 21205717 is about 4604.9665579676. The cubic root of 21205717 is about 276.7903769264.

Adding to 21205717 its reverse (71750212), we get a palindrome (92955929).

The spelling of 21205717 in words is "twenty-one million, two hundred five thousand, seven hundred seventeen".