21003665 has 4 divisors (see below), whose sum is σ = 25204404.
Its totient is φ = 16802928.
The previous prime is 21003659. The next prime is 21003667. The reversal of 21003665 is 56630012.
Adding to 21003665 its reverse (56630012), we get a palindrome (77633677).
It is a semiprime because it is the product of two primes.
It can be written as a sum of positive squares in 2 ways, for example, as 10297681 + 10705984 = 3209^2 + 3272^2
It is a cyclic number.
It is not a de Polignac number, because 21003665 - 26 = 21003601 is a prime.
It is a Duffinian number.
It is a self number, because there is not a number n which added to its sum of digits gives 21003665.
It is not an unprimeable number, because it can be changed into a prime (21003667) 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 3 ways as a sum of consecutive naturals, for example, 2100362 + ... + 2100371.
It is an arithmetic number, because the mean of its divisors is an integer number (6301101).
Almost surely, 221003665 is an apocalyptic number.
It is an amenable number.
21003665 is a deficient number, since it is larger than the sum of its proper divisors (4200739).
21003665 is an equidigital number, since it uses as much as digits as its factorization.
21003665 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 4200738.
The product of its (nonzero) digits is 1080, while the sum is 23.
The square root of 21003665 is about 4582.9755617939.
The cubic root of 21003665 is about 275.9084666364.
The spelling of 21003665 in words is "twenty-one million, three thousand, six hundred sixty-five".