640991 has 4 divisors (see below), whose sum is σ = 690312. Its totient is φ = 591672.

The previous prime is 640973. The next prime is 640993. The reversal of 640991 is 199046.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199046 = 2 ⋅99523.

It is a cyclic number.

It is not a de Polignac number, because 640991 - 2¹⁴ = 624607 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 (640993) 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, 24641 + ... + 24666.

It is an arithmetic number, because the mean of its divisors is an integer number (172578).

2⁶⁴⁰⁹⁹¹ is an apocalyptic number.

640991 is a deficient number, since it is larger than the sum of its proper divisors (49321).

640991 is a wasteful number, since it uses less digits than its factorization.

640991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 49320.

The product of its (nonzero) digits is 1944, while the sum is 29.

The square root of 640991 is about 800.6191354196. The cubic root of 640991 is about 86.2218447623.

The spelling of 640991 in words is "six hundred forty thousand, nine hundred ninety-one".