987991 has 2 divisors, whose sum is σ = 987992. Its totient is φ = 987990.

The previous prime is 987983. The next prime is 987997. The reversal of 987991 is 199789.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 987991 - 2³ = 987983 is a prime.

It is a super-4 number, since 4×987991⁴ (a number of 25 digits) contains 4444 as substring. Note that it is a super-d number also for d = 2.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (987997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 493995 + 493996.

It is an arithmetic number, because the mean of its divisors is an integer number (493996).

2⁹⁸⁷⁹⁹¹ is an apocalyptic number.

987991 is a deficient number, since it is larger than the sum of its proper divisors (1).

987991 is an equidigital number, since it uses as much as digits as its factorization.

987991 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 987991 is about 993.9773639274. The cubic root of 987991 is about 99.5980868221.

The spelling of 987991 in words is "nine hundred eighty-seven thousand, nine hundred ninety-one".