643981 has 4 divisors (see below), whose sum is σ = 693532. Its totient is φ = 594432.

The previous prime is 643969. The next prime is 643991. The reversal of 643981 is 189346.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 188356 + 455625 = 434^2 + 675^2 .

It is a cyclic number.

It is not a de Polignac number, because 643981 - 2⁵ = 643949 is a prime.

It is a super-2 number, since 2×643981² = 829423056722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (643961) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 24756 + ... + 24781.

It is an arithmetic number, because the mean of its divisors is an integer number (173383).

2⁶⁴³⁹⁸¹ is an apocalyptic number.

It is an amenable number.

643981 is a deficient number, since it is larger than the sum of its proper divisors (49551).

643981 is a wasteful number, since it uses less digits than its factorization.

643981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 49550.

The product of its digits is 5184, while the sum is 31.

The square root of 643981 is about 802.4842677586. The cubic root of 643981 is about 86.3557018145.

The spelling of 643981 in words is "six hundred forty-three thousand, nine hundred eighty-one".