21561985 has 8 divisors (see below), whose sum is σ = 26131176. Its totient is φ = 17078400.

The previous prime is 21561949. The next prime is 21561997. The reversal of 21561985 is 58916512.

It can be written as a sum of positive squares in 4 ways, for example, as 3448449 + 18113536 = 1857^2 + 4256^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21561985 - 2¹⁵ = 21529217 is a prime.

It is a Duffinian number.

It is an unprimeable number.

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 7 ways as a sum of consecutive naturals, for example, 20844 + ... + 21853.

It is an arithmetic number, because the mean of its divisors is an integer number (3266397).

Almost surely, 2^21561985 is an apocalyptic number.

It is an amenable number.

21561985 is a deficient number, since it is larger than the sum of its proper divisors (4569191).

21561985 is a wasteful number, since it uses less digits than its factorization.

21561985 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 42803.

The product of its digits is 21600, while the sum is 37.

The square root of 21561985 is about 4643.4884515846. The cubic root of 21561985 is about 278.3318544469.

The spelling of 21561985 in words is "twenty-one million, five hundred sixty-one thousand, nine hundred eighty-five".