645981 has 16 divisors (see below), whose sum is σ = 1036800. Its totient is φ = 349488.

The previous prime is 645979. The next prime is 646003. The reversal of 645981 is 189546.

It is not a de Polignac number, because 645981 - 2¹ = 645979 is a prime.

It is a super-2 number, since 2×645981² = 834582904722, which contains 22 as substring.

It is a Curzon number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 645981.

It is not an unprimeable number, because it can be changed into a prime (645901) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 411 + ... + 1208.

It is an arithmetic number, because the mean of its divisors is an integer number (64800).

2⁶⁴⁵⁹⁸¹ is an apocalyptic number.

It is an amenable number.

645981 is a deficient number, since it is larger than the sum of its proper divisors (390819).

645981 is a wasteful number, since it uses less digits than its factorization.

645981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1648.

The product of its digits is 8640, while the sum is 33.

The square root of 645981 is about 803.7294320852. The cubic root of 645981 is about 86.4450072008.

The spelling of 645981 in words is "six hundred forty-five thousand, nine hundred eighty-one".