21506161 has 4 divisors (see below), whose sum is σ = 21727972. Its totient is φ = 21284352.

The previous prime is 21506159. The next prime is 21506167. The reversal of 21506161 is 16160512.

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 11771761 + 9734400 = 3431^2 + 3120^2 .

It is a cyclic number.

It is not a de Polignac number, because 21506161 - 2¹ = 21506159 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21506167) 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, 110760 + ... + 110953.

It is an arithmetic number, because the mean of its divisors is an integer number (5431993).

Almost surely, 2^21506161 is an apocalyptic number.

It is an amenable number.

21506161 is a deficient number, since it is larger than the sum of its proper divisors (221811).

21506161 is an equidigital number, since it uses as much as digits as its factorization.

21506161 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 221810.

The product of its (nonzero) digits is 360, while the sum is 22.

The square root of 21506161 is about 4637.4735578761. The cubic root of 21506161 is about 278.0914463751.

Adding to 21506161 its reverse (16160512), we get a palindrome (37666673).

The spelling of 21506161 in words is "twenty-one million, five hundred six thousand, one hundred sixty-one".