21157121065 has 4 divisors (see below), whose sum is σ = 25388545284. Its totient is φ = 16925696848.

The previous prime is 21157121057. The next prime is 21157121099. The reversal of 21157121065 is 56012175112.

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 4189713984 + 16967407081 = 64728^2 + 130259^2 .

It is a cyclic number.

It is not a de Polignac number, because 21157121065 - 2³ = 21157121057 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2115712102 + ... + 2115712111.

It is an arithmetic number, because the mean of its divisors is an integer number (6347136321).

Almost surely, 2^21157121065 is an apocalyptic number.

It is an amenable number.

21157121065 is a deficient number, since it is larger than the sum of its proper divisors (4231424219).

21157121065 is an equidigital number, since it uses as much as digits as its factorization.

21157121065 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4231424218.

The product of its (nonzero) digits is 4200, while the sum is 31.

Adding to 21157121065 its reverse (56012175112), we get a palindrome (77169296177).

The spelling of 21157121065 in words is "twenty-one billion, one hundred fifty-seven million, one hundred twenty-one thousand, sixty-five".