1605985921 has 4 divisors (see below), whose sum is σ = 1616764500. Its totient is φ = 1595207344.

The previous prime is 1605985897. The next prime is 1605985981. The reversal of 1605985921 is 1295895061.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1295895061 = 1021 ⋅1269241.

It can be written as a sum of positive squares in 2 ways, for example, as 726033025 + 879952896 = 26945^2 + 29664^2 .

It is a cyclic number.

It is not a de Polignac number, because 1605985921 - 2⁵ = 1605985889 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1605985981) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5389066 + ... + 5389363.

It is an arithmetic number, because the mean of its divisors is an integer number (404191125).

Almost surely, 2^1605985921 is an apocalyptic number.

It is an amenable number.

1605985921 is a deficient number, since it is larger than the sum of its proper divisors (10778579).

1605985921 is a wasteful number, since it uses less digits than its factorization.

1605985921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10778578.

The product of its (nonzero) digits is 194400, while the sum is 46.

The square root of 1605985921 is about 40074.7541601942. The cubic root of 1605985921 is about 1171.0638583770.

The spelling of 1605985921 in words is "one billion, six hundred five million, nine hundred eighty-five thousand, nine hundred twenty-one".