721921 has 2 divisors, whose sum is σ = 721922. Its totient is φ = 721920.

The previous prime is 721909. The next prime is 721951. The reversal of 721921 is 129127.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 408321 + 313600 = 639^2 + 560^2 .

It is an emirp because it is prime and its reverse (129127) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 721921 - 2⁷ = 721793 is a prime.

It is a Chen prime.

It is a junction number, because it is equal to n+sod(n) for n = 721892 and 721901.

It is not a weakly prime, because it can be changed into another prime (721951) 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 as a sum of consecutive naturals, namely, 360960 + 360961.

It is an arithmetic number, because the mean of its divisors is an integer number (360961).

It is a Proth number, since it is equal to 705 ⋅ 2¹⁰ + 1 and 705 < 2¹⁰.

2⁷²¹⁹²¹ is an apocalyptic number.

It is an amenable number.

721921 is a deficient number, since it is larger than the sum of its proper divisors (1).

721921 is an equidigital number, since it uses as much as digits as its factorization.

721921 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 252, while the sum is 22.

The square root of 721921 is about 849.6593435019. The cubic root of 721921 is about 89.7077350572.

The spelling of 721921 in words is "seven hundred twenty-one thousand, nine hundred twenty-one".