21121256161 has 16 divisors (see below), whose sum is σ = 22896369360. Its totient is φ = 19410094080.

The previous prime is 21121256131. The next prime is 21121256209. The reversal of 21121256161 is 16165212112.

It can be written as a sum of positive squares in 8 ways, for example, as 80013025 + 21041243136 = 8945^2 + 145056^2 .

It is a de Polignac number, because none of the positive numbers 2^k-21121256161 is a prime.

It is a super-2 number, since 2×21121256161² (a number of 21 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21121256131) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 67735 + ... + 216403.

It is an arithmetic number, because the mean of its divisors is an integer number (1431023085).

Almost surely, 2^21121256161 is an apocalyptic number.

It is an amenable number.

21121256161 is a deficient number, since it is larger than the sum of its proper divisors (1775113199).

21121256161 is a wasteful number, since it uses less digits than its factorization.

21121256161 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 148884.

The product of its digits is 1440, while the sum is 28.

Adding to 21121256161 its reverse (16165212112), we get a palindrome (37286468273).

The spelling of 21121256161 in words is "twenty-one billion, one hundred twenty-one million, two hundred fifty-six thousand, one hundred sixty-one".