640981981 has 2 divisors, whose sum is σ = 640981982. Its totient is φ = 640981980.

The previous prime is 640981949. The next prime is 640982003. The reversal of 640981981 is 189189046.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 474804100 + 166177881 = 21790^2 + 12891^2 .

It is a cyclic number.

It is not a de Polignac number, because 640981981 - 2⁵ = 640981949 is a prime.

It is a super-2 number, since 2×640981981² = 821715799933368722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (640981921) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 320490990 + 320490991.

It is an arithmetic number, because the mean of its divisors is an integer number (320490991).

Almost surely, 2^640981981 is an apocalyptic number.

It is an amenable number.

640981981 is a deficient number, since it is larger than the sum of its proper divisors (1).

640981981 is an equidigital number, since it uses as much as digits as its factorization.

640981981 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 124416, while the sum is 46.

The square root of 640981981 is about 25317.6219459885. The cubic root of 640981981 is about 862.2144036847.

The spelling of 640981981 in words is "six hundred forty million, nine hundred eighty-one thousand, nine hundred eighty-one".