21100821 has 36 divisors (see below), whose sum is σ = 34588512. Its totient is φ = 11387376.

The previous prime is 21100813. The next prime is 21100823. The reversal of 21100821 is 12800112.

It is not a de Polignac number, because 21100821 - 2³ = 21100813 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21100821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21100823) by changing a digit.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 267060 + ... + 267138.

It is an arithmetic number, because the mean of its divisors is an integer number (960792).

Almost surely, 2^21100821 is an apocalyptic number.

21100821 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21100821 is a deficient number, since it is larger than the sum of its proper divisors (13487691).

21100821 is an equidigital number, since it uses as much as digits as its factorization.

21100821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 198 (or 112 counting only the distinct ones).

The product of its (nonzero) digits is 32, while the sum is 15.

The square root of 21100821 is about 4593.5629961937. The cubic root of 21100821 is about 276.3332327702. Note that the first 3 decimals are identical.

Adding to 21100821 its reverse (12800112), we get a palindrome (33900933).

The spelling of 21100821 in words is "twenty-one million, one hundred thousand, eight hundred twenty-one".