21457666561 has 4 divisors (see below), whose sum is σ = 21981024324. Its totient is φ = 20934308800.

The previous prime is 21457666523. The next prime is 21457666589. The reversal of 21457666561 is 16566675412.

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 5972352961 + 15485313600 = 77281^2 + 124440^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21457666561 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21457666661) 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, 261678820 + ... + 261678901.

It is an arithmetic number, because the mean of its divisors is an integer number (5495256081).

Almost surely, 2^21457666561 is an apocalyptic number.

It is an amenable number.

21457666561 is a deficient number, since it is larger than the sum of its proper divisors (523357763).

21457666561 is an equidigital number, since it uses as much as digits as its factorization.

21457666561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 523357762.

The product of its digits is 1814400, while the sum is 49.

The spelling of 21457666561 in words is "twenty-one billion, four hundred fifty-seven million, six hundred sixty-six thousand, five hundred sixty-one".