821857921 has 4 divisors (see below), whose sum is σ = 823907844. Its totient is φ = 819808000.

The previous prime is 821857913. The next prime is 821857943. The reversal of 821857921 is 129758128.

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 500014321 + 321843600 = 22361^2 + 17940^2 .

It is a cyclic number.

It is not a de Polignac number, because 821857921 - 2³ = 821857913 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (821857961) 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, 1024360 + ... + 1025161.

It is an arithmetic number, because the mean of its divisors is an integer number (205976961).

Almost surely, 2^821857921 is an apocalyptic number.

It is an amenable number.

821857921 is a deficient number, since it is larger than the sum of its proper divisors (2049923).

821857921 is a wasteful number, since it uses less digits than its factorization.

821857921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2049922.

The product of its digits is 80640, while the sum is 43.

The square root of 821857921 is about 28668.0644794866. The cubic root of 821857921 is about 936.6965379435.

The spelling of 821857921 in words is "eight hundred twenty-one million, eight hundred fifty-seven thousand, nine hundred twenty-one".