21312561 has 8 divisors (see below), whose sum is σ = 28439424. Its totient is φ = 14197040.

The previous prime is 21312539. The next prime is 21312587. The reversal of 21312561 is 16521312.

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 21312561 - 2¹⁵ = 21279793 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21312461) 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, 3715 + ... + 7511.

It is an arithmetic number, because the mean of its divisors is an integer number (3554928).

Almost surely, 2^21312561 is an apocalyptic number.

It is an amenable number.

21312561 is a deficient number, since it is larger than the sum of its proper divisors (7126863).

21312561 is a wasteful number, since it uses less digits than its factorization.

21312561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5671.

The product of its digits is 360, while the sum is 21.

The square root of 21312561 is about 4616.5529348205. The cubic root of 21312561 is about 277.2544634359.

Adding to 21312561 its reverse (16521312), we get a palindrome (37833873).

The spelling of 21312561 in words is "twenty-one million, three hundred twelve thousand, five hundred sixty-one".