721639 has 6 divisors (see below), whose sum is σ = 762000. Its totient is φ = 683316.

The previous prime is 721631. The next prime is 721661. The reversal of 721639 is 936127.

721639 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is not a de Polignac number, because 721639 - 2³ = 721631 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (721631) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 639 + ... + 1360.

It is an arithmetic number, because the mean of its divisors is an integer number (127000).

2⁷²¹⁶³⁹ is an apocalyptic number.

721639 is a deficient number, since it is larger than the sum of its proper divisors (40361).

721639 is a wasteful number, since it uses less digits than its factorization.

721639 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2037 (or 2018 counting only the distinct ones).

The product of its digits is 2268, while the sum is 28.

The square root of 721639 is about 849.4933784321. The cubic root of 721639 is about 89.6960528575.

It can be divided in two parts, 72 and 1639, that added together give a triangular number (1711 = T₅₈).

The spelling of 721639 in words is "seven hundred twenty-one thousand, six hundred thirty-nine".