1987991 has 2 divisors, whose sum is σ = 1987992. Its totient is φ = 1987990.

The previous prime is 1987987. The next prime is 1988011. The reversal of 1987991 is 1997891.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 1987991 - 2² = 1987987 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1987901) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 993995 + 993996.

It is an arithmetic number, because the mean of its divisors is an integer number (993996).

2^1987991 is an apocalyptic number.

1987991 is a deficient number, since it is larger than the sum of its proper divisors (1).

1987991 is an equidigital number, since it uses as much as digits as its factorization.

1987991 is an odious number, because the sum of its binary digits is odd.

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

The square root of 1987991 is about 1409.9613469879. The cubic root of 1987991 is about 125.7394253762.

The spelling of 1987991 in words is "one million, nine hundred eighty-seven thousand, nine hundred ninety-one".