991981 has 2 divisors, whose sum is σ = 991982. Its totient is φ = 991980.

The previous prime is 991979. The next prime is 991987. The reversal of 991981 is 189199.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 980100 + 11881 = 990^2 + 109^2 .

It is an emirp because it is prime and its reverse (189199) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 991981 - 2¹ = 991979 is a prime.

It is a super-2 number, since 2×991981² = 1968052608722, which contains 22 as substring.

Together with 991979, it forms a pair of twin primes.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 991981.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495990 + 495991.

It is an arithmetic number, because the mean of its divisors is an integer number (495991).

2⁹⁹¹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

991981 is a deficient number, since it is larger than the sum of its proper divisors (1).

991981 is an equidigital number, since it uses as much as digits as its factorization.

991981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5832, while the sum is 37.

The square root of 991981 is about 995.9824295639. The cubic root of 991981 is about 99.7319823069.

It can be divided in two parts, 99198 and 1, that added together give a palindrome (99199).

The spelling of 991981 in words is "nine hundred ninety-one thousand, nine hundred eighty-one".