640721 has 8 divisors (see below), whose sum is σ = 664224. Its totient is φ = 617760.

The previous prime is 640691. The next prime is 640727. The reversal of 640721 is 127046.

640721 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 640721 - 2¹⁰ = 639697 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 640693 and 640702.

It is not an unprimeable number, because it can be changed into a prime (640727) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4826 + ... + 4956.

It is an arithmetic number, because the mean of its divisors is an integer number (83028).

2⁶⁴⁰⁷²¹ is an apocalyptic number.

It is an amenable number.

640721 is a deficient number, since it is larger than the sum of its proper divisors (23503).

640721 is a wasteful number, since it uses less digits than its factorization.

640721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 271.

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

The square root of 640721 is about 800.4504981571. The cubic root of 640721 is about 86.2097368609.

Adding to 640721 its reverse (127046), we get a palindrome (767767).

The spelling of 640721 in words is "six hundred forty thousand, seven hundred twenty-one".