21365665021 has 4 divisors (see below), whose sum is σ = 21886778844. Its totient is φ = 20844551200.

The previous prime is 21365665019. The next prime is 21365665043. The reversal of 21365665021 is 12056656312.

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 2208060100 + 19157604921 = 46990^2 + 138411^2 .

It is a cyclic number.

It is not a de Polignac number, because 21365665021 - 2¹ = 21365665019 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21365660021) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 260556850 + ... + 260556931.

It is an arithmetic number, because the mean of its divisors is an integer number (5471694711).

Almost surely, 2^21365665021 is an apocalyptic number.

It is an amenable number.

21365665021 is a deficient number, since it is larger than the sum of its proper divisors (521113823).

21365665021 is an equidigital number, since it uses as much as digits as its factorization.

21365665021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 521113822.

The product of its (nonzero) digits is 64800, while the sum is 37.

The spelling of 21365665021 in words is "twenty-one billion, three hundred sixty-five million, six hundred sixty-five thousand, twenty-one".