21311561 has 8 divisors (see below), whose sum is σ = 21762720. Its totient is φ = 20864448.

The previous prime is 21311527. The next prime is 21311599. The reversal of 21311561 is 16511312.

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

It is a cyclic number.

It is not a de Polignac number, because 21311561 - 2⁶ = 21311497 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21311161) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 11105 + ... + 12881.

It is an arithmetic number, because the mean of its divisors is an integer number (2720340).

Almost surely, 2^21311561 is an apocalyptic number.

It is an amenable number.

21311561 is a deficient number, since it is larger than the sum of its proper divisors (451159).

21311561 is a wasteful number, since it uses less digits than its factorization.

21311561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2023.

The product of its digits is 180, while the sum is 20.

The square root of 21311561 is about 4616.4446276328. Note that the first 3 decimals coincide. The cubic root of 21311561 is about 277.2501270448.

Adding to 21311561 its reverse (16511312), we get a palindrome (37822873).

The spelling of 21311561 in words is "twenty-one million, three hundred eleven thousand, five hundred sixty-one".