1801991 has 4 divisors (see below), whose sum is σ = 1845984. Its totient is φ = 1758000.

The previous prime is 1801967. The next prime is 1801997. The reversal of 1801991 is 1991081.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1991081 = 37 ⋅53813.

It is a cyclic number.

It is not a de Polignac number, because 1801991 - 2⁶ = 1801927 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1801997) 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, 21935 + ... + 22016.

It is an arithmetic number, because the mean of its divisors is an integer number (461496).

2^1801991 is an apocalyptic number.

1801991 is a deficient number, since it is larger than the sum of its proper divisors (43993).

1801991 is an equidigital number, since it uses as much as digits as its factorization.

1801991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 43992.

The product of its (nonzero) digits is 648, while the sum is 29.

The square root of 1801991 is about 1342.3825833197. The cubic root of 1801991 is about 121.6888739931.

The spelling of 1801991 in words is "one million, eight hundred one thousand, nine hundred ninety-one".