1661881991 has 32 divisors (see below), whose sum is σ = 2128972800. Its totient is φ = 1285956000.

The previous prime is 1661881961. The next prime is 1661882021. The reversal of 1661881991 is 1991881661.

It is an interprime number because it is at equal distance from previous prime (1661881961) and next prime (1661882021).

It is a de Polignac number, because none of the positive numbers 2^k-1661881991 is a prime.

1661881991 is a strobogrammatic number because it is the same when read upside-down.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1661881961) 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, 568265 + ... + 571181.

It is an arithmetic number, because the mean of its divisors is an integer number (66530400).

Almost surely, 2^1661881991 is an apocalyptic number.

1661881991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

1661881991 is a deficient number, since it is larger than the sum of its proper divisors (467090809).

1661881991 is a wasteful number, since it uses less digits than its factorization.

1661881991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3100 (or 3086 counting only the distinct ones).

The product of its digits is 186624, while the sum is 50.

The square root of 1661881991 is about 40766.1868587191. The cubic root of 1661881991 is about 1184.4954419937.

The spelling of 1661881991 in words is "one billion, six hundred sixty-one million, eight hundred eighty-one thousand, nine hundred ninety-one".