21091021 has 12 divisors (see below), whose sum is σ = 24611232. Its totient is φ = 18021528.

The previous prime is 21091009. The next prime is 21091051. The reversal of 21091021 is 12019012.

It is not a de Polignac number, because 21091021 - 2¹¹ = 21088973 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21090986 and 21091004.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21091051) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 25216 + ... + 26038.

It is an arithmetic number, because the mean of its divisors is an integer number (2050936).

Almost surely, 2^21091021 is an apocalyptic number.

It is an amenable number.

21091021 is a deficient number, since it is larger than the sum of its proper divisors (3520211).

21091021 is an equidigital number, since it uses as much as digits as its factorization.

21091021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1360 (or 1353 counting only the distinct ones).

The product of its (nonzero) digits is 36, while the sum is 16.

The square root of 21091021 is about 4592.4961622194. The cubic root of 21091021 is about 276.2904463618.

The spelling of 21091021 in words is "twenty-one million, ninety-one thousand, twenty-one".