721640 has 16 divisors (see below), whose sum is σ = 1623780. Its totient is φ = 288640.

The previous prime is 721631. The next prime is 721661. The reversal of 721640 is 46127.

It can be written as a sum of positive squares in 2 ways, for example, as 630436 + 91204 = 794^2 + 302^2 .

It is a Harshad number since it is a multiple of its sum of digits (20).

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 8981 + ... + 9060.

2⁷²¹⁶⁴⁰ is an apocalyptic number.

It is an amenable number.

721640 is an abundant number, since it is smaller than the sum of its proper divisors (902140).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

721640 is a wasteful number, since it uses less digits than its factorization.

721640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18052 (or 18048 counting only the distinct ones).

The product of its (nonzero) digits is 336, while the sum is 20.

The square root of 721640 is about 849.4939670180. The cubic root of 721640 is about 89.6960942891.

Adding to 721640 its reverse (46127), we get a palindrome (767767).

The spelling of 721640 in words is "seven hundred twenty-one thousand, six hundred forty".