21206566405 has 4 divisors (see below), whose sum is σ = 25447879692. Its totient is φ = 16965253120.

The previous prime is 21206566399. The next prime is 21206566417. The reversal of 21206566405 is 50466560212.

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 3030942916 + 18175623489 = 55054^2 + 134817^2 .

It is not a de Polignac number, because 21206566405 - 2¹⁵ = 21206533637 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2120656636 + ... + 2120656645.

It is an arithmetic number, because the mean of its divisors is an integer number (6361969923).

Almost surely, 2^21206566405 is an apocalyptic number.

It is an amenable number.

21206566405 is a deficient number, since it is larger than the sum of its proper divisors (4241313287).

21206566405 is an equidigital number, since it uses as much as digits as its factorization.

21206566405 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4241313286.

The product of its (nonzero) digits is 86400, while the sum is 37.

The spelling of 21206566405 in words is "twenty-one billion, two hundred six million, five hundred sixty-six thousand, four hundred five".