821699991 has 16 divisors (see below), whose sum is σ = 1391238400. Its totient is φ = 469542744.

The previous prime is 821699987. The next prime is 821700017. The reversal of 821699991 is 199996128.

It is not a de Polignac number, because 821699991 - 2² = 821699987 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (821698991) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2173621 + ... + 2173998.

It is an arithmetic number, because the mean of its divisors is an integer number (86952400).

Almost surely, 2^821699991 is an apocalyptic number.

821699991 is a deficient number, since it is larger than the sum of its proper divisors (569538409).

821699991 is a wasteful number, since it uses less digits than its factorization.

821699991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4347635 (or 4347629 counting only the distinct ones).

The product of its digits is 629856, while the sum is 54.

The square root of 821699991 is about 28665.3098884348. The cubic root of 821699991 is about 936.6365348874.

The spelling of 821699991 in words is "eight hundred twenty-one million, six hundred ninety-nine thousand, nine hundred ninety-one".