21000561 has 16 divisors (see below), whose sum is σ = 28693440. Its totient is φ = 13657280.

The previous prime is 21000557. The next prime is 21000563. The reversal of 21000561 is 16500012.

It is a cyclic number.

It is not a de Polignac number, because 21000561 - 2² = 21000557 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21000561.

It is not an unprimeable number, because it can be changed into a prime (21000563) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 42526 + ... + 43016.

It is an arithmetic number, because the mean of its divisors is an integer number (1793340).

Almost surely, 2^21000561 is an apocalyptic number.

It is an amenable number.

21000561 is a deficient number, since it is larger than the sum of its proper divisors (7692879).

21000561 is a wasteful number, since it uses less digits than its factorization.

21000561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 816.

The product of its (nonzero) digits is 60, while the sum is 15.

The square root of 21000561 is about 4582.6369046653. The cubic root of 21000561 is about 275.8948743725.

Adding to 21000561 its reverse (16500012), we get a palindrome (37500573).

The spelling of 21000561 in words is "twenty-one million, five hundred sixty-one", and thus it is an aban number.