991861261 has 2 divisors, whose sum is σ = 991861262. Its totient is φ = 991861260.

The previous prime is 991861259. The next prime is 991861279. The reversal of 991861261 is 162168199.

991861261 = 22269² + 22270².

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 495952900 + 495908361 = 22270^2 + 22269^2 .

It is an emirp because it is prime and its reverse (162168199) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 991861261 - 2¹ = 991861259 is a prime.

It is a super-2 number, since 2×991861261² = 1967577522145020242, which contains 22 as substring.

Together with 991861259, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991861861) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495930630 + 495930631.

It is an arithmetic number, because the mean of its divisors is an integer number (495930631).

Almost surely, 2^991861261 is an apocalyptic number.

991861261 is the 22270-th centered square number.

It is an amenable number.

991861261 is a deficient number, since it is larger than the sum of its proper divisors (1).

991861261 is an equidigital number, since it uses as much as digits as its factorization.

991861261 is an evil number, because the sum of its binary digits is even.

The product of its digits is 46656, while the sum is 43.

The square root of 991861261 is about 31493.8289352057. The cubic root of 991861261 is about 997.2796936435.

The spelling of 991861261 in words is "nine hundred ninety-one million, eight hundred sixty-one thousand, two hundred sixty-one".