1681991665 has 4 divisors (see below), whose sum is σ = 2018390004. Its totient is φ = 1345593328.

The previous prime is 1681991659. The next prime is 1681991671. The reversal of 1681991665 is 5661991861.

It is a happy number.

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

It is an interprime number because it is at equal distance from previous prime (1681991659) and next prime (1681991671).

It can be written as a sum of positive squares in 2 ways, for example, as 1031180544 + 650811121 = 32112^2 + 25511^2 .

It is a cyclic number.

It is not a de Polignac number, because 1681991665 - 2³ = 1681991657 is a prime.

It is a super-2 number, since 2×1681991665² = 5658191922258944450, which contains 22 as substring.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 168199162 + ... + 168199171.

It is an arithmetic number, because the mean of its divisors is an integer number (504597501).

Almost surely, 2^1681991665 is an apocalyptic number.

It is an amenable number.

1681991665 is a deficient number, since it is larger than the sum of its proper divisors (336398339).

1681991665 is an equidigital number, since it uses as much as digits as its factorization.

1681991665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 336398338.

The product of its digits is 699840, while the sum is 52.

The square root of 1681991665 is about 41012.0916925728. The cubic root of 1681991665 is about 1189.2539788952.

The spelling of 1681991665 in words is "one billion, six hundred eighty-one million, nine hundred ninety-one thousand, six hundred sixty-five".