3991661 has 4 divisors (see below), whose sum is σ = 3997692. Its totient is φ = 3985632.

The previous prime is 3991649. The next prime is 3991667. The reversal of 3991661 is 1661993.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1661993 = 43 ⋅38651.

It can be written as a sum of positive squares in 2 ways, for example, as 2220100 + 1771561 = 1490^2 + 1331^2 .

It is a cyclic number.

It is not a de Polignac number, because 3991661 - 2¹⁴ = 3975277 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (3991667) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1880 + ... + 3393.

It is an arithmetic number, because the mean of its divisors is an integer number (999423).

Almost surely, 2^3991661 is an apocalyptic number.

It is an amenable number.

3991661 is a deficient number, since it is larger than the sum of its proper divisors (6031).

3991661 is an equidigital number, since it uses as much as digits as its factorization.

3991661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6030.

The product of its digits is 8748, while the sum is 35.

The square root of 3991661 is about 1997.9141623203. The cubic root of 3991661 is about 158.6297173061.

The spelling of 3991661 in words is "three million, nine hundred ninety-one thousand, six hundred sixty-one".