1991584781 has 4 divisors (see below), whose sum is σ = 1995085500. Its totient is φ = 1988084064.

The previous prime is 1991584741. The next prime is 1991584813. The reversal of 1991584781 is 1874851991.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 1343882281 + 647702500 = 36659^2 + 25450^2 .

It is a cyclic number.

It is not a de Polignac number, because 1991584781 - 2¹⁰ = 1991583757 is a prime.

It is a super-2 number, since 2×1991584781² = 7932819879821635922, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991584741) 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, 1749506 + ... + 1750643.

It is an arithmetic number, because the mean of its divisors is an integer number (498771375).

Almost surely, 2^1991584781 is an apocalyptic number.

It is an amenable number.

1991584781 is a deficient number, since it is larger than the sum of its proper divisors (3500719).

1991584781 is an equidigital number, since it uses as much as digits as its factorization.

1991584781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3500718.

The product of its digits is 725760, while the sum is 53.

The square root of 1991584781 is about 44627.1753643450. The cubic root of 1991584781 is about 1258.1514804240.

The spelling of 1991584781 in words is "one billion, nine hundred ninety-one million, five hundred eighty-four thousand, seven hundred eighty-one".