1991518777 has 4 divisors (see below), whose sum is σ = 1995019380. Its totient is φ = 1988018176.

The previous prime is 1991518759. The next prime is 1991518783. The reversal of 1991518777 is 7778151991.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 7778151991 = 239 ⋅32544569.

It can be written as a sum of positive squares in 2 ways, for example, as 1297512441 + 694006336 = 36021^2 + 26344^2 .

It is a cyclic number.

It is not a de Polignac number, because 1991518777 - 2¹¹ = 1991516729 is a prime.

It is a super-2 number, since 2×1991518777² = 7932294078287151458, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1991518757) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1749448 + ... + 1750585.

It is an arithmetic number, because the mean of its divisors is an integer number (498754845).

Almost surely, 2^1991518777 is an apocalyptic number.

It is an amenable number.

1991518777 is a deficient number, since it is larger than the sum of its proper divisors (3500603).

1991518777 is an equidigital number, since it uses as much as digits as its factorization.

1991518777 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3500602.

The product of its digits is 1111320, while the sum is 55.

The square root of 1991518777 is about 44626.4358536507. The cubic root of 1991518777 is about 1258.1375812839.

The spelling of 1991518777 in words is "one billion, nine hundred ninety-one million, five hundred eighteen thousand, seven hundred seventy-seven".