21205561 has 8 divisors (see below), whose sum is σ = 22901760. Its totient is φ = 19518672.

The previous prime is 21205559. The next prime is 21205573. The reversal of 21205561 is 16550212.

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 21205561 - 2¹ = 21205559 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21205361) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2850 + ... + 7108.

It is an arithmetic number, because the mean of its divisors is an integer number (2862720).

Almost surely, 2^21205561 is an apocalyptic number.

It is an amenable number.

21205561 is a deficient number, since it is larger than the sum of its proper divisors (1696199).

21205561 is a wasteful number, since it uses less digits than its factorization.

21205561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4655.

The product of its (nonzero) digits is 600, while the sum is 22.

The square root of 21205561 is about 4604.9496197027. The cubic root of 21205561 is about 276.7896981879.

Adding to 21205561 its reverse (16550212), we get a palindrome (37755773).

The spelling of 21205561 in words is "twenty-one million, two hundred five thousand, five hundred sixty-one".