21681161 has 4 divisors (see below), whose sum is σ = 21691152. Its totient is φ = 21671172.

The previous prime is 21681151. The next prime is 21681167. The reversal of 21681161 is 16118612.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21681161 - 2⁶ = 21681097 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21681161.

It is not an unprimeable number, because it can be changed into a prime (21681167) 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, 215 + ... + 6588.

It is an arithmetic number, because the mean of its divisors is an integer number (5422788).

Almost surely, 2^21681161 is an apocalyptic number.

It is an amenable number.

21681161 is a deficient number, since it is larger than the sum of its proper divisors (9991).

21681161 is an equidigital number, since it uses as much as digits as its factorization.

21681161 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9990.

The product of its digits is 576, while the sum is 26.

The square root of 21681161 is about 4656.3033621103. The cubic root of 21681161 is about 278.8437051490.

Adding to 21681161 its reverse (16118612), we get a palindrome (37799773).

The spelling of 21681161 in words is "twenty-one million, six hundred eighty-one thousand, one hundred sixty-one".