21681355609 has 4 divisors (see below), whose sum is σ = 21682132020. Its totient is φ = 21680579200.

The previous prime is 21681355589. The next prime is 21681355613. The reversal of 21681355609 is 90655318612.

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 9057709584 + 12623646025 = 95172^2 + 112355^2 .

It is a cyclic number.

It is not a de Polignac number, because 21681355609 - 2⁹ = 21681355097 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21681355649) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 344692 + ... + 402709.

It is an arithmetic number, because the mean of its divisors is an integer number (5420533005).

Almost surely, 2^21681355609 is an apocalyptic number.

It is an amenable number.

21681355609 is a deficient number, since it is larger than the sum of its proper divisors (776411).

21681355609 is an equidigital number, since it uses as much as digits as its factorization.

21681355609 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 776410.

The product of its (nonzero) digits is 388800, while the sum is 46.

The spelling of 21681355609 in words is "twenty-one billion, six hundred eighty-one million, three hundred fifty-five thousand, six hundred nine".