1991988 has 18 divisors (see below), whose sum is σ = 5035394. Its totient is φ = 663984.

The previous prime is 1991963. The next prime is 1991989. The reversal of 1991988 is 8891991.

It can be written as a sum of positive squares in only one way, i.e., 1411344 + 580644 = 1188^2 + 762^2 .

It is a tau number, because it is divible by the number of its divisors (18).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991989) 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 5 ways as a sum of consecutive naturals, for example, 27631 + ... + 27702.

2^1991988 is an apocalyptic number.

1991988 is a gapful number since it is divisible by the number (18) formed by its first and last digit.

It is an amenable number.

1991988 is an abundant number, since it is smaller than the sum of its proper divisors (3043406).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

1991988 is a wasteful number, since it uses less digits than its factorization.

1991988 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 55343 (or 55338 counting only the distinct ones).

The product of its digits is 46656, while the sum is 45.

The square root of 1991988 is about 1411.3780499923. The cubic root of 1991988 is about 125.8236383722.

It can be divided in two parts, 199 and 1988, that added together give a 7-th power (2187 = 3⁷).

The spelling of 1991988 in words is "one million, nine hundred ninety-one thousand, nine hundred eighty-eight".