639999991 has 4 divisors (see below), whose sum is σ = 660645184. Its totient is φ = 619354800.

The previous prime is 639999967. The next prime is 640000019. The reversal of 639999991 is 199999936.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 639999991 - 2⁵ = 639999959 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 (639999911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10322550 + ... + 10322611.

It is an arithmetic number, because the mean of its divisors is an integer number (165161296).

Almost surely, 2^639999991 is an apocalyptic number.

639999991 is a deficient number, since it is larger than the sum of its proper divisors (20645193).

639999991 is a wasteful number, since it uses less digits than its factorization.

639999991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20645192.

The product of its digits is 9565938, while the sum is 64.

The square root of 639999991 is about 25298.2211034689. The cubic root of 639999991 is about 861.7738719732.

It can be divided in two parts, 63999999 and 1, that added together give a 6-th power (64000000 = 20⁶).

The spelling of 639999991 in words is "six hundred thirty-nine million, nine hundred ninety-nine thousand, nine hundred ninety-one".