21728065 has 16 divisors (see below), whose sum is σ = 26941392. Its totient is φ = 16809984.

The previous prime is 21728051. The next prime is 21728071. The reversal of 21728065 is 56082712.

21728065 is nontrivially palindromic in base 16.

It can be written as a sum of positive squares in 8 ways, for example, as 8832784 + 12895281 = 2972^2 + 3591^2 .

It is a cyclic number.

It is not a de Polignac number, because 21728065 - 2⁹ = 21727553 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 (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 47317 + ... + 47773.

It is an arithmetic number, because the mean of its divisors is an integer number (1683837).

Almost surely, 2^21728065 is an apocalyptic number.

It is an amenable number.

21728065 is a deficient number, since it is larger than the sum of its proper divisors (5213327).

21728065 is a wasteful number, since it uses less digits than its factorization.

21728065 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 756.

The product of its (nonzero) digits is 6720, while the sum is 31.

The square root of 21728065 is about 4661.3372544797. The cubic root of 21728065 is about 279.0446394233.

It can be divided in two parts, 217 and 28065, that added together give a palindrome (28282).

The spelling of 21728065 in words is "twenty-one million, seven hundred twenty-eight thousand, sixty-five".