21636665 has 16 divisors (see below), whose sum is σ = 27600912. Its totient is φ = 16226304.

The previous prime is 21636661. The next prime is 21636677. The reversal of 21636665 is 56663612.

It can be written as a sum of positive squares in 8 ways, for example, as 32761 + 21603904 = 181^2 + 4648^2 .

It is a cyclic number.

It is not a de Polignac number, because 21636665 - 2² = 21636661 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21636661) 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 15 ways as a sum of consecutive naturals, for example, 38567 + ... + 39123.

It is an arithmetic number, because the mean of its divisors is an integer number (1725057).

Almost surely, 2^21636665 is an apocalyptic number.

It is an amenable number.

21636665 is a deficient number, since it is larger than the sum of its proper divisors (5964247).

21636665 is a wasteful number, since it uses less digits than its factorization.

21636665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1036.

The product of its digits is 38880, while the sum is 35.

The square root of 21636665 is about 4651.5228689108. The cubic root of 21636665 is about 278.6528185836.

The spelling of 21636665 in words is "twenty-one million, six hundred thirty-six thousand, six hundred sixty-five".