1644138981 has 12 divisors (see below), whose sum is σ = 2385332352. Its totient is φ = 1091262696.

The previous prime is 1644138961. The next prime is 1644138983. The reversal of 1644138981 is 1898314461.

It is not a de Polignac number, because 1644138981 - 2²³ = 1635750373 is a prime.

It is a super-3 number, since 3×1644138981³ (a number of 29 digits) contains 333 as substring. Note that it is a super-d number also for d = 2.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1644138983) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 400341 + ... + 404426.

It is an arithmetic number, because the mean of its divisors is an integer number (198777696).

Almost surely, 2^1644138981 is an apocalyptic number.

It is an amenable number.

1644138981 is a deficient number, since it is larger than the sum of its proper divisors (741193371).

1644138981 is a wasteful number, since it uses less digits than its factorization.

1644138981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 805000 (or 804997 counting only the distinct ones).

The product of its digits is 165888, while the sum is 45.

The square root of 1644138981 is about 40547.9836859985. The cubic root of 1644138981 is about 1180.2649460132.

It can be divided in two parts, 1644 and 138981, that added together give a square (140625 = 375²).

The spelling of 1644138981 in words is "one billion, six hundred forty-four million, one hundred thirty-eight thousand, nine hundred eighty-one".