821191681 has 4 divisors (see below), whose sum is σ = 869497092. Its totient is φ = 772886272.

The previous prime is 821191663. The next prime is 821191739. The reversal of 821191681 is 186191128.

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 250937281 + 570254400 = 15841^2 + 23880^2 .

It is a cyclic number.

It is not a de Polignac number, because 821191681 - 2¹⁵ = 821158913 is a prime.

It is a super-2 number, since 2×821191681² = 1348711553887211522, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 821191681.

It is not an unprimeable number, because it can be changed into a prime (821191601) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 24152680 + ... + 24152713.

It is an arithmetic number, because the mean of its divisors is an integer number (217374273).

Almost surely, 2^821191681 is an apocalyptic number.

It is an amenable number.

821191681 is a deficient number, since it is larger than the sum of its proper divisors (48305411).

821191681 is a wasteful number, since it uses less digits than its factorization.

821191681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 48305410.

The product of its digits is 6912, while the sum is 37.

The square root of 821191681 is about 28656.4422250914. The cubic root of 821191681 is about 936.4433581686.

The spelling of 821191681 in words is "eight hundred twenty-one million, one hundred ninety-one thousand, six hundred eighty-one".