9781991 has 2 divisors, whose sum is σ = 9781992. Its totient is φ = 9781990.

The previous prime is 9781963. The next prime is 9781997. The reversal of 9781991 is 1991879.

9781991 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a strong prime.

It is an emirp because it is prime and its reverse (1991879) is a distict prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-9781991 is a prime.

It is a Sophie Germain prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (9781997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 4890995 + 4890996.

It is an arithmetic number, because the mean of its divisors is an integer number (4890996).

Almost surely, 2^9781991 is an apocalyptic number.

9781991 is a deficient number, since it is larger than the sum of its proper divisors (1).

9781991 is an equidigital number, since it uses as much as digits as its factorization.

9781991 is an evil number, because the sum of its binary digits is even.

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

The square root of 9781991 is about 3127.6174638213. The cubic root of 9781991 is about 213.8663313505.

The spelling of 9781991 in words is "nine million, seven hundred eighty-one thousand, nine hundred ninety-one".