721891 has 2 divisors, whose sum is σ = 721892. Its totient is φ = 721890.

The previous prime is 721883. The next prime is 721909. The reversal of 721891 is 198127.

721891 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a weak prime.

It is an emirp because it is prime and its reverse (198127) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 721891 - 2³ = 721883 is a prime.

It is a super-2 number, since 2×721891² = 1042253231762, which contains 22 as substring.

It is a Chen prime.

It is equal to p₅₈₁₇₇ and since 721891 and 58177 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (721291) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360945 + 360946.

It is an arithmetic number, because the mean of its divisors is an integer number (360946).

2⁷²¹⁸⁹¹ is an apocalyptic number.

721891 is a deficient number, since it is larger than the sum of its proper divisors (1).

721891 is an equidigital number, since it uses as much as digits as its factorization.

721891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1008, while the sum is 28.

The square root of 721891 is about 849.6416891843. The cubic root of 721891 is about 89.7064924146.

It can be divided in two parts, 721 and 891, that multiplied together give a triangular number (642411 = T₁₁₃₃).

The spelling of 721891 in words is "seven hundred twenty-one thousand, eight hundred ninety-one".