21081561 has 8 divisors (see below), whose sum is σ = 28252224. Its totient is φ = 13982640.

The previous prime is 21081559. The next prime is 21081569. The reversal of 21081561 is 16518012.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21081561 - 2¹ = 21081559 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21081569) by changing a digit.

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 7 ways as a sum of consecutive naturals, for example, 17245 + ... + 18426.

It is an arithmetic number, because the mean of its divisors is an integer number (3531528).

Almost surely, 2^21081561 is an apocalyptic number.

It is an amenable number.

21081561 is a deficient number, since it is larger than the sum of its proper divisors (7170663).

21081561 is a wasteful number, since it uses less digits than its factorization.

21081561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 35871.

The product of its (nonzero) digits is 480, while the sum is 24.

The square root of 21081561 is about 4591.4661057227. The cubic root of 21081561 is about 276.2491318059.

Adding to 21081561 its reverse (16518012), we get a palindrome (37599573).

The spelling of 21081561 in words is "twenty-one million, eighty-one thousand, five hundred sixty-one".