216921 has 4 divisors (see below), whose sum is σ = 289232. Its totient is φ = 144612.

The previous prime is 216919. The next prime is 216947. The reversal of 216921 is 129612.

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 not a de Polignac number, because 216921 - 2¹ = 216919 is a prime.

It is a D-number.

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

It is a Duffinian number.

216921 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (216901) 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, 36151 + ... + 36156.

It is an arithmetic number, because the mean of its divisors is an integer number (72308).

2²¹⁶⁹²¹ is an apocalyptic number.

It is an amenable number.

216921 is a deficient number, since it is larger than the sum of its proper divisors (72311).

216921 is an equidigital number, since it uses as much as digits as its factorization.

216921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 72310.

The product of its digits is 216, while the sum is 21.

The square root of 216921 is about 465.7477858240. The cubic root of 216921 is about 60.0851568591.

The spelling of 216921 in words is "two hundred sixteen thousand, nine hundred twenty-one".