217680721 has 4 divisors (see below), whose sum is σ = 218128192. Its totient is φ = 217233252.

The previous prime is 217680713. The next prime is 217680779. The reversal of 217680721 is 127086712.

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 217680721 - 2³ = 217680713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (217680821) 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, 223005 + ... + 223978.

It is an arithmetic number, because the mean of its divisors is an integer number (54532048).

Almost surely, 2^217680721 is an apocalyptic number.

It is an amenable number.

217680721 is a deficient number, since it is larger than the sum of its proper divisors (447471).

217680721 is an equidigital number, since it uses as much as digits as its factorization.

217680721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 447470.

The product of its (nonzero) digits is 9408, while the sum is 34.

The square root of 217680721 is about 14754.0069472669. The cubic root of 217680721 is about 601.5522041227.

The spelling of 217680721 in words is "two hundred seventeen million, six hundred eighty thousand, seven hundred twenty-one".