639781 has 4 divisors (see below), whose sum is σ = 648864. Its totient is φ = 630700.

The previous prime is 639757. The next prime is 639833. The reversal of 639781 is 187936.

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 a cyclic number.

It is not a de Polignac number, because 639781 - 2⁹ = 639269 is a prime.

It is a super-2 number, since 2×639781² = 818639455922, which contains 22 as substring.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (639701) 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, 4435 + ... + 4576.

It is an arithmetic number, because the mean of its divisors is an integer number (162216).

2⁶³⁹⁷⁸¹ is an apocalyptic number.

It is an amenable number.

639781 is a deficient number, since it is larger than the sum of its proper divisors (9083).

639781 is an equidigital number, since it uses as much as digits as its factorization.

639781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9082.

The product of its digits is 9072, while the sum is 34.

The square root of 639781 is about 799.8631132888. The cubic root of 639781 is about 86.1675568716.

The spelling of 639781 in words is "six hundred thirty-nine thousand, seven hundred eighty-one".