991987 has 2 divisors, whose sum is σ = 991988. Its totient is φ = 991986.

The previous prime is 991981. The next prime is 991999. The reversal of 991987 is 789199.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 991987 - 2³ = 991979 is a prime.

It is not a weakly prime, because it can be changed into another prime (991981) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495993 + 495994.

It is an arithmetic number, because the mean of its divisors is an integer number (495994).

2⁹⁹¹⁹⁸⁷ is an apocalyptic number.

991987 is a deficient number, since it is larger than the sum of its proper divisors (1).

991987 is an equidigital number, since it uses as much as digits as its factorization.

991987 is an evil number, because the sum of its binary digits is even.

The product of its digits is 40824, while the sum is 43.

The square root of 991987 is about 995.9854416607. The cubic root of 991987 is about 99.7321833829.

The spelling of 991987 in words is "nine hundred ninety-one thousand, nine hundred eighty-seven".