21102020021 has 4 divisors (see below), whose sum is σ = 21102411372. Its totient is φ = 21101628672.

The previous prime is 21102019957. The next prime is 21102020053. The reversal of 21102020021 is 12002020112.

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 12282180625 + 8819839396 = 110825^2 + 93914^2 .

It is a cyclic number.

It is not a de Polignac number, because 21102020021 - 2⁶ = 21102019957 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21102019984 and 21102020011.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21102070021) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 98810 + ... + 227963.

It is an arithmetic number, because the mean of its divisors is an integer number (5275602843).

Almost surely, 2^21102020021 is an apocalyptic number.

It is an amenable number.

21102020021 is a deficient number, since it is larger than the sum of its proper divisors (391351).

21102020021 is an equidigital number, since it uses as much as digits as its factorization.

21102020021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 391350.

The product of its (nonzero) digits is 16, while the sum is 11.

Adding to 21102020021 its reverse (12002020112), we get a palindrome (33104040133).

Subtracting from 21102020021 its reverse (12002020112), we obtain a palindrome (9099999909).

The spelling of 21102020021 in words is "twenty-one billion, one hundred two million, twenty thousand, twenty-one".