21005653 has 4 divisors (see below), whose sum is σ = 21518028. Its totient is φ = 20493280.

The previous prime is 21005651. The next prime is 21005669. The reversal of 21005653 is 35650012.

It is a happy number.

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 1096209 + 19909444 = 1047^2 + 4462^2 .

It is a cyclic number.

It is not a de Polignac number, because 21005653 - 2¹ = 21005651 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005651) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 256126 + ... + 256207.

It is an arithmetic number, because the mean of its divisors is an integer number (5379507).

Almost surely, 2^21005653 is an apocalyptic number.

It is an amenable number.

21005653 is a deficient number, since it is larger than the sum of its proper divisors (512375).

21005653 is an equidigital number, since it uses as much as digits as its factorization.

21005653 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 512374.

The product of its (nonzero) digits is 900, while the sum is 22.

The square root of 21005653 is about 4583.1924463195. The cubic root of 21005653 is about 275.9171712875.

Adding to 21005653 its reverse (35650012), we get a palindrome (56655665).

The spelling of 21005653 in words is "twenty-one million, five thousand, six hundred fifty-three".