1991816278201 has 2 divisors, whose sum is σ = 1991816278202. Its totient is φ = 1991816278200.

The previous prime is 1991816278147. The next prime is 1991816278211. The reversal of 1991816278201 is 1028726181991.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1750067055801 + 241749222400 = 1322901^2 + 491680^2 .

It is a cyclic number.

It is not a de Polignac number, because 1991816278201 - 2⁷ = 1991816278073 is a prime.

It is a super-2 number, since 2×1991816278201² (a number of 25 digits) contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (1991816278211) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 995908139100 + 995908139101.

It is an arithmetic number, because the mean of its divisors is an integer number (995908139101).

Almost surely, 2^{1991816278201} is an apocalyptic number.

It is an amenable number.

1991816278201 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991816278201 is an equidigital number, since it uses as much as digits as its factorization.

1991816278201 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 870912, while the sum is 55.

The spelling of 1991816278201 in words is "one trillion, nine hundred ninety-one billion, eight hundred sixteen million, two hundred seventy-eight thousand, two hundred one".