1991987 has 4 divisors (see below), whose sum is σ = 1996536. Its totient is φ = 1987440.

The previous prime is 1991963. The next prime is 1991989. The reversal of 1991987 is 7891991.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 1991987 - 2¹² = 1987891 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1991989) 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, 1538 + ... + 2519.

It is an arithmetic number, because the mean of its divisors is an integer number (499134).

2^1991987 is an apocalyptic number.

1991987 is a deficient number, since it is larger than the sum of its proper divisors (4549).

1991987 is an equidigital number, since it uses as much as digits as its factorization.

1991987 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4548.

The product of its digits is 40824, while the sum is 44.

The square root of 1991987 is about 1411.3776957285. The cubic root of 1991987 is about 125.8236173172.

The spelling of 1991987 in words is "one million, nine hundred ninety-one thousand, nine hundred eighty-seven".