640406721 has 4 divisors (see below), whose sum is σ = 853875632. Its totient is φ = 426937812.

The previous prime is 640406699. The next prime is 640406749. The reversal of 640406721 is 127604046.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 640406721 - 2⁹ = 640406209 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (640406521) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 106734451 + ... + 106734456.

It is an arithmetic number, because the mean of its divisors is an integer number (213468908).

Almost surely, 2^640406721 is an apocalyptic number.

It is an amenable number.

640406721 is a deficient number, since it is larger than the sum of its proper divisors (213468911).

640406721 is a wasteful number, since it uses less digits than its factorization.

640406721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 213468910.

The product of its (nonzero) digits is 8064, while the sum is 30.

The square root of 640406721 is about 25306.2585342045. The cubic root of 640406721 is about 861.9563902371.

The spelling of 640406721 in words is "six hundred forty million, four hundred six thousand, seven hundred twenty-one".