20931721 has 2 divisors, whose sum is σ = 20931722. Its totient is φ = 20931720.

The previous prime is 20931719. The next prime is 20931731. The reversal of 20931721 is 12713902.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19404025 + 1527696 = 4405^2 + 1236^2 .

It is a cyclic number.

It is not a de Polignac number, because 20931721 - 2¹ = 20931719 is a prime.

Together with 20931719, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (20931731) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10465860 + 10465861.

It is an arithmetic number, because the mean of its divisors is an integer number (10465861).

Almost surely, 2^20931721 is an apocalyptic number.

It is an amenable number.

20931721 is a deficient number, since it is larger than the sum of its proper divisors (1).

20931721 is an equidigital number, since it uses as much as digits as its factorization.

20931721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 756, while the sum is 25.

The square root of 20931721 is about 4575.1197798528. The cubic root of 20931721 is about 275.5930825353.

The spelling of 20931721 in words is "twenty million, nine hundred thirty-one thousand, seven hundred twenty-one".