21100021 has 2 divisors, whose sum is σ = 21100022. Its totient is φ = 21100020.

The previous prime is 21100019. The next prime is 21100033. The reversal of 21100021 is 12000112.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20520900 + 579121 = 4530^2 + 761^2 .

It is a cyclic number.

It is not a de Polignac number, because 21100021 - 2¹ = 21100019 is a prime.

Together with 21100019, it forms a pair of twin primes.

It is a junction number, because it is equal to n+sod(n) for n = 21099977 and 21100013.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21103021) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10550010 + 10550011.

It is an arithmetic number, because the mean of its divisors is an integer number (10550011).

Almost surely, 2^21100021 is an apocalyptic number.

It is an amenable number.

21100021 is a deficient number, since it is larger than the sum of its proper divisors (1).

21100021 is an equidigital number, since it uses as much as digits as its factorization.

21100021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 4, while the sum is 7.

The square root of 21100021 is about 4593.4759169936. The cubic root of 21100021 is about 276.3297404988.

Adding to 21100021 its reverse (12000112), we get a palindrome (33100133).

Subtracting from 21100021 its reverse (12000112), we obtain a palindrome (9099909).

The spelling of 21100021 in words is "twenty-one million, one hundred thousand, twenty-one".