1661991 has 8 divisors (see below), whose sum is σ = 2246640. Its totient is φ = 1092672.

The previous prime is 1661983. The next prime is 1662007. The reversal of 1661991 is 1991661.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 1661991 - 2³ = 1661983 is a prime.

1661991 is a strobogrammatic number because it is the same when read upside-down.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1666991) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3576 + ... + 4013.

It is an arithmetic number, because the mean of its divisors is an integer number (280830).

2^1661991 is an apocalyptic number.

1661991 is a deficient number, since it is larger than the sum of its proper divisors (584649).

1661991 is an equidigital number, since it uses as much as digits as its factorization.

1661991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7665.

The product of its digits is 2916, while the sum is 33.

The square root of 1661991 is about 1289.1822989787. The cubic root of 1661991 is about 118.4521339910.

The spelling of 1661991 in words is "one million, six hundred sixty-one thousand, nine hundred ninety-one".