1991808021 has 32 divisors (see below), whose sum is σ = 3077867520. Its totient is φ = 1122208704.

The previous prime is 1991807989. The next prime is 1991808023. The reversal of 1991808021 is 1208081991.

It is not a de Polignac number, because 1991808021 - 2⁵ = 1991807989 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991808023) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 1276840 + ... + 1278398.

It is an arithmetic number, because the mean of its divisors is an integer number (96183360).

Almost surely, 2^1991808021 is an apocalyptic number.

It is an amenable number.

1991808021 is a deficient number, since it is larger than the sum of its proper divisors (1086059499).

1991808021 is a wasteful number, since it uses less digits than its factorization.

1991808021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2385.

The product of its (nonzero) digits is 10368, while the sum is 39.

The square root of 1991808021 is about 44629.6764608483. The cubic root of 1991808021 is about 1258.1984880877.

The spelling of 1991808021 in words is "one billion, nine hundred ninety-one million, eight hundred eight thousand, twenty-one".