639981 has 10 divisors (see below), whose sum is σ = 956142. Its totient is φ = 426600.

The previous prime is 639959. The next prime is 639983. The reversal of 639981 is 189936.

It can be written as a sum of positive squares in only one way, i.e., 585225 + 54756 = 765^2 + 234^2 .

It is not a de Polignac number, because 639981 - 2⁵ = 639949 is a prime.

It is a super-2 number, since 2×639981² = 819151360722, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (639983) by changing a digit.

It is a polite number, since it can be written in 9 ways as a sum of consecutive naturals, for example, 3870 + ... + 4031.

2⁶³⁹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

639981 is a deficient number, since it is larger than the sum of its proper divisors (316161).

639981 is an equidigital number, since it uses as much as digits as its factorization.

639981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7913 (or 7904 counting only the distinct ones).

The product of its digits is 11664, while the sum is 36.

The square root of 639981 is about 799.9881249119. The cubic root of 639981 is about 86.1765347958.

The spelling of 639981 in words is "six hundred thirty-nine thousand, nine hundred eighty-one".