991660781 has 2 divisors, whose sum is σ = 991660782. Its totient is φ = 991660780.

The previous prime is 991660757. The next prime is 991660811. The reversal of 991660781 is 187066199.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 570015625 + 421645156 = 23875^2 + 20534^2 .

It is a cyclic number.

It is not a de Polignac number, because 991660781 - 2²⁶ = 924551917 is a prime.

It is a super-2 number, since 2×991660781² = 1966782209147059922, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991660741) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495830390 + 495830391.

It is an arithmetic number, because the mean of its divisors is an integer number (495830391).

Almost surely, 2^991660781 is an apocalyptic number.

It is an amenable number.

991660781 is a deficient number, since it is larger than the sum of its proper divisors (1).

991660781 is an equidigital number, since it uses as much as digits as its factorization.

991660781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 163296, while the sum is 47.

The square root of 991660781 is about 31490.6459285928. The cubic root of 991660781 is about 997.2124973823.

The spelling of 991660781 in words is "nine hundred ninety-one million, six hundred sixty thousand, seven hundred eighty-one".