31721 has 2 divisors, whose sum is σ = 31722. Its totient is φ = 31720.

The previous prime is 31699. The next prime is 31723. The reversal of 31721 is 12713.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18496 + 13225 = 136^2 + 115^2 .

It is an emirp because it is prime and its reverse (12713) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 31721 - 2⁶ = 31657 is a prime.

It is a Sophie Germain prime.

Together with 31723, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 31696 and 31705.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 31721.

It is not a weakly prime, because it can be changed into another prime (31723) 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 as a sum of consecutive naturals, namely, 15860 + 15861.

It is an arithmetic number, because the mean of its divisors is an integer number (15861).

2³¹⁷²¹ is an apocalyptic number.

It is an amenable number.

31721 is a deficient number, since it is larger than the sum of its proper divisors (1).

31721 is an equidigital number, since it uses as much as digits as its factorization.

31721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 42, while the sum is 14.

The square root of 31721 is about 178.1039022593. The cubic root of 31721 is about 31.6554838938.

The spelling of 31721 in words is "thirty-one thousand, seven hundred twenty-one".