3991840165681 has 27 divisors (see below), whose sum is σ = 4268322166017. Its totient is φ = 3732507085440.

The previous prime is 3991840165673. The next prime is 3991840165721. The reversal of 3991840165681 is 1865610481993.

The square root of 3991840165681 is 1997959.

It is a perfect power (a square), and thus also a powerful number.

3991840165681 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It can be written as a sum of positive squares in 4 ways, for example, as 141218875681 + 3850621290000 = 375791^2 + 1962300^2 .

It is not a de Polignac number, because 3991840165681 - 2³ = 3991840165673 is a prime.

It is a super-2 number, since 2×3991840165681² (a number of 26 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (3991840165601) by changing a digit.

It is a polite number, since it can be written in 26 ways as a sum of consecutive naturals, for example, 7166678655 + ... + 7166679211.

Almost surely, 2^{3991840165681} is an apocalyptic number.

3991840165681 is the 1997959-th square number.

3991840165681 is the 998980-th centered octagonal number.

It is an amenable number.

3991840165681 is a deficient number, since it is larger than the sum of its proper divisors (276482000336).

3991840165681 is an frugal number, since it uses more digits than its factorization.

3991840165681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1570 (or 785 counting only the distinct ones).

The product of its (nonzero) digits is 11197440, while the sum is 61.

It can be divided in two parts, 3991840 and 165681, that added together give a square (4157521 = 2039²).

The spelling of 3991840165681 in words is "three trillion, nine hundred ninety-one billion, eight hundred forty million, one hundred sixty-five thousand, six hundred eighty-one".