16721 has 4 divisors (see below), whose sum is σ = 17472. Its totient is φ = 15972.

The previous prime is 16703. The next prime is 16729. The reversal of 16721 is 12761.

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, and also an emirpimes, since its reverse is a distinct semiprime: 12761 = 7 ⋅1823.

It is a cyclic number.

It is not a de Polignac number, because 16721 - 2⁶ = 16657 is a prime.

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

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 16721.

It is not an unprimeable number, because it can be changed into a prime (16729) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 341 + ... + 386.

It is an arithmetic number, because the mean of its divisors is an integer number (4368).

2¹⁶⁷²¹ is an apocalyptic number.

It is an amenable number.

16721 is a deficient number, since it is larger than the sum of its proper divisors (751).

16721 is an equidigital number, since it uses as much as digits as its factorization.

16721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 750.

The product of its digits is 84, while the sum is 17.

The square root of 16721 is about 129.3097057455. The cubic root of 16721 is about 25.5713750684.

It can be divided in two parts, 16 and 721, that added together give a palindrome (737).

The spelling of 16721 in words is "sixteen thousand, seven hundred twenty-one".