21186561 has 16 divisors (see below), whose sum is σ = 30965760. Its totient is φ = 12778320.

The previous prime is 21186553. The next prime is 21186563. The reversal of 21186561 is 16568112.

It is not a de Polignac number, because 21186561 - 2³ = 21186553 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21186563) 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, 5920 + ... + 8798.

It is an arithmetic number, because the mean of its divisors is an integer number (1935360).

Almost surely, 2^21186561 is an apocalyptic number.

It is an amenable number.

21186561 is a deficient number, since it is larger than the sum of its proper divisors (9779199).

21186561 is a wasteful number, since it uses less digits than its factorization.

21186561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3116.

The product of its digits is 2880, while the sum is 30.

The square root of 21186561 is about 4602.8861597915. The cubic root of 21186561 is about 276.7070064242.

The spelling of 21186561 in words is "twenty-one million, one hundred eighty-six thousand, five hundred sixty-one".