216721 has 4 divisors (see below), whose sum is σ = 223744. Its totient is φ = 209700.

The previous prime is 216719. The next prime is 216731. The reversal of 216721 is 127612.

216721 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

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 216721 - 2¹ = 216719 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (216731) 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, 3465 + ... + 3526.

It is an arithmetic number, because the mean of its divisors is an integer number (55936).

2²¹⁶⁷²¹ is an apocalyptic number.

It is an amenable number.

216721 is a deficient number, since it is larger than the sum of its proper divisors (7023).

216721 is an equidigital number, since it uses as much as digits as its factorization.

216721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7022.

The product of its digits is 168, while the sum is 19.

The square root of 216721 is about 465.5330278294. The cubic root of 216721 is about 60.0666851167.

It can be divided in two parts, 21 and 6721, that multiplied together give a palindrome (141141).

The spelling of 216721 in words is "two hundred sixteen thousand, seven hundred twenty-one".