21781665 has 12 divisors (see below), whose sum is σ = 37754964. Its totient is φ = 11616864.

The previous prime is 21781649. The next prime is 21781673. The reversal of 21781665 is 56618712.

It can be written as a sum of positive squares in 2 ways, for example, as 16136289 + 5645376 = 4017^2 + 2376^2 .

It is not a de Polignac number, because 21781665 - 2⁴ = 21781649 is a prime.

It is an unprimeable number.

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 11 ways as a sum of consecutive naturals, for example, 241974 + ... + 242063.

It is an arithmetic number, because the mean of its divisors is an integer number (3146247).

Almost surely, 2^21781665 is an apocalyptic number.

It is an amenable number.

21781665 is a deficient number, since it is larger than the sum of its proper divisors (15973299).

21781665 is a wasteful number, since it uses less digits than its factorization.

21781665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 484048 (or 484045 counting only the distinct ones).

The product of its digits is 20160, while the sum is 36.

The square root of 21781665 is about 4667.0831361783. The cubic root of 21781665 is about 279.2739052867.

The spelling of 21781665 in words is "twenty-one million, seven hundred eighty-one thousand, six hundred sixty-five".