21207217 has 4 divisors (see below), whose sum is σ = 21226356. Its totient is φ = 21188080.

The previous prime is 21207167. The next prime is 21207223. The reversal of 21207217 is 71270212.

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 20711601 + 495616 = 4551^2 + 704^2 .

It is a cyclic number.

It is not a de Polignac number, because 21207217 - 2¹¹ = 21205169 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21237217) 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, 7798 + ... + 10159.

It is an arithmetic number, because the mean of its divisors is an integer number (5306589).

Almost surely, 2^21207217 is an apocalyptic number.

It is an amenable number.

21207217 is a deficient number, since it is larger than the sum of its proper divisors (19139).

21207217 is a wasteful number, since it uses less digits than its factorization.

21207217 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 19138.

The product of its (nonzero) digits is 392, while the sum is 22.

The square root of 21207217 is about 4605.1294227198. The cubic root of 21207217 is about 276.7969030875.

Adding to 21207217 its reverse (71270212), we get a palindrome (92477429).

The spelling of 21207217 in words is "twenty-one million, two hundred seven thousand, two hundred seventeen".