21205643 has 4 divisors (see below), whose sum is σ = 21889728. Its totient is φ = 20521560.

The previous prime is 21205637. The next prime is 21205649. The reversal of 21205643 is 34650212.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21205637) and next prime (21205649).

It is a cyclic number.

It is not a de Polignac number, because 21205643 - 2¹⁰ = 21204619 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21205649) 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, 341996 + ... + 342057.

It is an arithmetic number, because the mean of its divisors is an integer number (5472432).

Almost surely, 2^21205643 is an apocalyptic number.

21205643 is a deficient number, since it is larger than the sum of its proper divisors (684085).

21205643 is an equidigital number, since it uses as much as digits as its factorization.

21205643 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 684084.

The product of its (nonzero) digits is 1440, while the sum is 23.

The square root of 21205643 is about 4604.9585231574. The cubic root of 21205643 is about 276.7900549611.

Adding to 21205643 its reverse (34650212), we get a palindrome (55855855).

The spelling of 21205643 in words is "twenty-one million, two hundred five thousand, six hundred forty-three".