640731 has 16 divisors (see below), whose sum is σ = 1051904. Its totient is φ = 337824.

The previous prime is 640727. The next prime is 640733. The reversal of 640731 is 137046.

It is not a de Polignac number, because 640731 - 2² = 640727 is a prime.

It is a super-2 number, since 2×640731² = 821072428722, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a junction number, because it is equal to n+sod(n) for n = 640698 and 640707.

It is not an unprimeable number, because it can be changed into a prime (640733) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 901 + ... + 1446.

It is an arithmetic number, because the mean of its divisors is an integer number (65744).

2⁶⁴⁰⁷³¹ is an apocalyptic number.

640731 is a deficient number, since it is larger than the sum of its proper divisors (411173).

640731 is a wasteful number, since it uses less digits than its factorization.

640731 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2370.

The product of its (nonzero) digits is 504, while the sum is 21.

The square root of 640731 is about 800.4567446152. The cubic root of 640731 is about 86.2101853623.

Adding to 640731 its reverse (137046), we get a palindrome (777777).

The spelling of 640731 in words is "six hundred forty thousand, seven hundred thirty-one".