21012561 has 16 divisors (see below), whose sum is σ = 32961600. Its totient is φ = 13184064.

The previous prime is 21012533. The next prime is 21012569. The reversal of 21012561 is 16521012.

21012561 is a `hidden beast` number, since 2 + 101 + 2 + 561 = 666.

It is not a de Polignac number, because 21012561 - 2¹⁰ = 21011537 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is not an unprimeable number, because it can be changed into a prime (21012569) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 22431 + ... + 23348.

It is an arithmetic number, because the mean of its divisors is an integer number (2060100).

Almost surely, 2^21012561 is an apocalyptic number.

It is an amenable number.

21012561 is a deficient number, since it is larger than the sum of its proper divisors (11949039).

21012561 is a wasteful number, since it uses less digits than its factorization.

21012561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 45805 (or 45799 counting only the distinct ones).

The product of its (nonzero) digits is 120, while the sum is 18.

The square root of 21012561 is about 4583.9460075354. The cubic root of 21012561 is about 275.9474143673.

Adding to 21012561 its reverse (16521012), we get a palindrome (37533573).

The spelling of 21012561 in words is "twenty-one million, twelve thousand, five hundred sixty-one".