1645813681 has 4 divisors (see below), whose sum is σ = 1646201700. Its totient is φ = 1645425664.

The previous prime is 1645813679. The next prime is 1645813703. The reversal of 1645813681 is 1863185461.

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 758727025 + 887086656 = 27545^2 + 29784^2 .

It is a cyclic number.

It is not a de Polignac number, because 1645813681 - 2¹ = 1645813679 is a prime.

It is a super-4 number, since 4×1645813681⁴ (a number of 38 digits) contains 4444 as substring. Note that it is a super-d number also for d = 2.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1645843681) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 187576 + ... + 196153.

It is an arithmetic number, because the mean of its divisors is an integer number (411550425).

Almost surely, 2^1645813681 is an apocalyptic number.

It is an amenable number.

1645813681 is a deficient number, since it is larger than the sum of its proper divisors (388019).

1645813681 is an equidigital number, since it uses as much as digits as its factorization.

1645813681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 388018.

The product of its digits is 138240, while the sum is 43.

The square root of 1645813681 is about 40568.6292718894. The cubic root of 1645813681 is about 1180.6655445427.

The spelling of 1645813681 in words is "one billion, six hundred forty-five million, eight hundred thirteen thousand, six hundred eighty-one".