19721 has 8 divisors (see below), whose sum is σ = 22344. Its totient is φ = 17280.

The previous prime is 19717. The next prime is 19727. The reversal of 19721 is 12791.

19721 = 14² + 15² + ... + 39².

19721 = 4⁶ + 5⁶.

It can be written as a sum of positive squares in 4 ways, for example, as 400 + 19321 = 20^2 + 139^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 19721 - 2² = 19717 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 19693 and 19702.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (19727) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 461 + ... + 501.

It is an arithmetic number, because the mean of its divisors is an integer number (2793).

2¹⁹⁷²¹ is an apocalyptic number.

It is an amenable number.

19721 is a deficient number, since it is larger than the sum of its proper divisors (2623).

19721 is a wasteful number, since it uses less digits than its factorization.

19721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 91.

The product of its digits is 126, while the sum is 20.

The square root of 19721 is about 140.4314779528. The cubic root of 19721 is about 27.0173642304.

Subtracting from 19721 its sum of digits (20), we obtain a triangular number (19701 = T₁₉₈).

The spelling of 19721 in words is "nineteen thousand, seven hundred twenty-one".