639569981 has 2 divisors, whose sum is σ = 639569982. Its totient is φ = 639569980.

The previous prime is 639569969. The next prime is 639569999. The reversal of 639569981 is 189965936.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 430189081 + 209380900 = 20741^2 + 14470^2 .

It is a cyclic number.

It is not a de Polignac number, because 639569981 - 2²² = 635375677 is a prime.

It is a super-2 number, since 2×639569981² = 818099521192680722, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (639569911) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 319784990 + 319784991.

It is an arithmetic number, because the mean of its divisors is an integer number (319784991).

Almost surely, 2^639569981 is an apocalyptic number.

It is an amenable number.

639569981 is a deficient number, since it is larger than the sum of its proper divisors (1).

639569981 is an equidigital number, since it uses as much as digits as its factorization.

639569981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3149280, while the sum is 56.

The square root of 639569981 is about 25289.7208565061. The cubic root of 639569981 is about 861.5808227996.

It can be divided in two parts, 63956 and 9981, that added together give a palindrome (73937).

The spelling of 639569981 in words is "six hundred thirty-nine million, five hundred sixty-nine thousand, nine hundred eighty-one".