1661691 has 4 divisors (see below), whose sum is σ = 2215592. Its totient is φ = 1107792.

The previous prime is 1661677. The next prime is 1661713. The reversal of 1661691 is 1961661.

It is a happy number.

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

It is not a de Polignac number, because 1661691 - 2⁵ = 1661659 is a prime.

It is a super-2 number, since 2×1661691² = 5522433958962, which contains 22 as substring.

It is a D-number.

It is a Duffinian number.

1661691 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (1661641) 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, 276946 + ... + 276951.

It is an arithmetic number, because the mean of its divisors is an integer number (553898).

2^1661691 is an apocalyptic number.

1661691 is a deficient number, since it is larger than the sum of its proper divisors (553901).

1661691 is an equidigital number, since it uses as much as digits as its factorization.

1661691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 553900.

The product of its digits is 1944, while the sum is 30.

The square root of 1661691 is about 1289.0659409045. The cubic root of 1661691 is about 118.4450064396.

Subtracting from 1661691 its sum of digits (30), we obtain a palindrome (1661661).

The spelling of 1661691 in words is "one million, six hundred sixty-one thousand, six hundred ninety-one".